Unfortunately the game's devs didn't make a particularily good job explaining game mechanics and gameplay details, so this guide is one of the attempts to correct that oversight.

There are two abbreviations in this text.
D - Damage.
DR - Damage Resistance.
There could be more but I don't feel like it's really necessary.
* - means the piece of equipment already described above.

Now let's talk about the only thing you certainly shouldn't do playing the game for the first time - don't choose the Extreme difficulty. I saw some guides where it was recommended, just to save time because theoretically you can get all the achievements during one playthrough. And theoretically it's possible, but it won't be fun at all. It's the worst thing you can do to yourself and no amount of saved time is worth endless frustration you would bring upon yourself.

The second worst choice for your first playthrough is Hard difficulty. It's possible to finish the game on Hard, but there are two reasons you don't wanna do it.
First, there is about five really tough fights which are hard even on Medium difficulty. To win them you need to know game mechanics pretty well and you won't be able to properly learn everything you need to know until at least the middle of the game.
Second, there is no respec in any shape or form in the game. So theoretically you can screw your characters up to the point where you won't be able to ever win the final battle.
So I suggest the Medium difficulty for your first playthrough. There are not many differences between Hard and Extreme and you can skip Hard altogether. And also the game is pretty short anyway, don't deprive yourself of some fun.

Arkail

• Strength 14
  
• Agility 6
  
• Stamina 15
  
• Mind 5

Gladiator's greaves [DR +5%, Stability +3]
Slicer [D +5]

Styx

• Strength 5
  
• Agility 15
  
• Stamina 6
  
• Mind 14

He starts with complete Survivor's set.
Survivor's leather [DR +6%, Life reg./s +1, Stamina +2]
Survivor's gloves [DR +2%, Striking force +3]
Survivor's leggins [DR +2%, Stability +3]
Sacrificial daggers [D +3, Converts 10% of inflicted D into Life points]
Sometimes Styx can get Throat slitters [D +3, Dodge +5%] instead of Sacrificial daggers.

On Easy and Medium difficulty you can play however you want using any skills in any order. But there are only that many really useful skills for Hard and Extreme difficulties. The combination that worked really well for me was defensive and tanky Arkail and Styx attacking mostly from a distance.

Every attack has one of three impact levels - Light, Medium or Heavy. If enemies' Stability is low enough the attack can stagger them for a breif moment, which could be very useful, especially for flimsy Styx.
Let's look at all the skills they have first.

Increases damage and allows Arkail to use the Bloodjaws' most powerful attack.

• Damage: +50%.
  
• Damage reduction: -50%.
  
• Deflection: -50%.

Upgrades:

• Every critical hit will stun the target [3];
  
• Every successful hit has a 25% chance of causing bleeding [3].

Fast Attack
Default attack. Automatically executed when Arkail fights in Offensive stance.

• Damage: 100%. Type of impact: Medium.

Upgrades:

• Inflicts bleeding to any enemy affected by a vulnerability [3];
  
• Increase stability by 100% during the attack [3].

Powerful Blow
Heavy vertical attack that mere mortals cannot parry but, which can be dodged more easily than a normal attack.

• Damage: 125%.
  
• Number of blows: 3.
  
• Type of impact; Heavy.
  
• Enemy parry: -50%,
  
• Enemy dodge: +50%.

Upgrades:

• The powerful Blow is impossible to interrupt [5];
  
• Damage is increased by 50% [5].

Feint
Feint attack difficult to dodge, but with slight impact and more likely to be parried.

• Damage: 150%.
  
• Number of blows: 2.
  
• Type of impact: Light.
  
• Enemy parry: +50%,
  
• Enemy dodge: -75%.

Upgrades:

• The attack has a 100% chance of disabling the target [5];
  
• Damage is increased by 50% [5].

Round Attack [2]
Double strike pushing back all enemies within range. Very powerful, but can be parried or dodged.

• Area: 4 meters.
  
• Damage: 100%.
  
• Number of blows: 2.
  
• Type of impact: Medium.
  
• Enemy parry: +50%,
  
• Enemy dodge: +50%.

Upgrade:

• The Round Attack cannot be interrupted [6];
  
• Takes away the enemies' bonus to dodging or parrying the attack [6].

Sworn Enemy [2]
Series of two heavy blows that generate a high threat for the target.

• Damage: 125%.
  
• Number of blows: 2.
  
• Type of impact: Heavy.
  
• Threat: +100.

Upgrades:

• The attack restores 30 Life points [6];
  
• The attack increases rage by 20 points [6].

Offensive Charge [6]
Charges at a distant enemy to land a very powerful blow, inflicting heavy damage.

• Damage: 200%.
  
• Type of impact: Heavy.

Upgrades:

• The impact has a 50% chance to stun a target[9];
  
• Damage is increased by 100% [9].

Murderous Rage [10]
Arkail lowers his guard to concentrate on the attack, inflicting more damage at the cost of his defense.

• Duration: 30 sec.
  
• Damage: +50%.
  
• Damage reduction: -100%.
  
• Deflection: -100%.

Upgrades:

• Chances of landing critical hit increase by 10% [13];
  
• Defense penalty (armor and deflection) is decreased to -0% [13].

Raises the chances of Deflection by 20%, slows down the Rage increase and allows Arkail to automatically riposte some of the deflected attacks.

• Rage per sec.: -2.
  
• Deflection: +25%.
  
• Counter-attack: +35%.

Upgrades:

• The chance of deflecting increases by 10% in Defensive stance [3];
  
• Arkail's rage increases even more slowly [3].

Quick Attack
Default attack. Executed in the form of an automatic riposte when Arkail successfully counter-attacks.

• Damage: 100%.
  
• Type of impact: Medium.

Upgrades:

• The attack has a 25% chance of stunning the target [3];
  
• The target's chances of parrying or dodging the attack are reduced by 25% [3].

Stunning punch
Powerful punch that stuns the target, who becomes incapable of reacting for several seconds.

• Damage: 150%.
  
• Type of impact: Heavy.
  
• Chance of stunning: 75%.
  
• Enemy parry: +50%.
  
• Enemy dodge: +50%.

Upgrades:

• The target's chances of parrying of dodging the attack are reduced by 50% [5];
  
• Damage is increased by 50% [5].

Rage Release
Decreases the rage gauge over a short period of time.

• Duration: 3 sec.
  
• Rage per second: -5.

Upgrades:

• The decrease of Rage is amplified by 100% [5];
  
• Converts 100% of the suppressed rage into Life points [5].

Taunt [2]
Makes nearby enemies perceive a threat, depending how far away they are.

• Area: 8 meters.
  
• Threat: +100.

Upgrades:

• The Taunt has a 25% chance of destabilizing every provoked foe [6];
  
• Extends the area of effect of the Taunt by 100% [6].

Back and Force [2]
Fast attack that interrupts and knocks back a target.

• Damage: 50%.
  
• Number of blows: 3.
  
• Type of impact: Heavy.
  
• Striking force: 500%.

Upgrades:

• The attack has a 100% chance of stunning the target [6];
  
• Damage is increased by 100% [6].

Carapace [6]
Goes into a defensive stance, improving Damage reduction and Deflection while reducing offensive abilities (damage and critical hits) for 15 sec.

• Duration: 15 sec.
  
• Damage reduction: +25%.
  
• Deflection: +15%.
  
• Damage: -75%.
  
• Critical hits: -75%.

Upgrades:

• Increases the chances of counter-attacking with each deflected blow by 35% [9];
  
• Increases health regeneration by 50% [9].

Disengagement [10]
Area attack knocking back enemies, inflicting more damage if the targets are vulnerable.

• Area: 4 meters.
  
• Damage: 75%.
  
• Number of blows: 4.
  
• Type of impact: Heavy.
  
• Striking force: 150%.

Upgrades:

• The attack is impossible to interrupt [13];
  
• Increases striking force by 100% [13].

The number in square brackets denotes the level on which a skill can be obtained. If there is no number - the skill is available from the start. BTW starting skills of both characters are described in the official Manual.

Sheathes the knives and uses throwing daggers.
Upgrades:

• Increases Dodge by 15% [3];
  
• Increases health regeneration by 50% in Range Combat stance [3].

Throw Knife
Default attack. Automatically executed when Styx fights in Range Combat stance.

• Damage: 75%.
  
• Type of impact: Light.

Upgrades:

• The attack has 25% chance of causing vulnerability [3];
  
• Every critical hit increases concentration by 10 [3].

Accurate Throw
Expert throw causing greater damage with a high chance of interrupting the target.

• Concentration cost: 30.
  
• Damage: 225%.
  
• Type of impact: Light.
  
• Striking force: 200%.

Upgrades:

• Each critical hit has a 50% chance of stunning the target [5];
  
• The attack has a 25% chance of causing a vulnerability [5].

Low Blow
Kick that pushes the target back slightly with a high chance of interrupting him.

• Concentration cost: 15.
  
• Damage: 75%.
  
• Type of impact: Medium.
  
• Striking force: 500%.
  
• Pushes back: 2 meters.

Upgrades:

• The attack has a 50% chance of stunning the target [5];
  
• The distance the target is pushed back is increased by 100% [5].

Sharpshooter [2]
Concentrates on the targets, increasing the damage cost and the chances of a critical hit, but decreases the dodging ability.

• Concentration cost: 50.
  
• Duration: 20 sec.
  
• Damage: +50%.
  
• Critical hits: +50%.
  
• Dodge: -100%.

Upgrades:

• The chance of dodging is no longer decreased [6];
  
• Every critical hit has a 50% chance of stunning the target [6].

Beard Trim [2]
An explicit gesture that has the advantage of both enraging the target and reducing the threat generated for the other enemies.

• Threat to the target: +100.
  
• Threat to others: -50.

Upgrades:

• Increases the generated threat to the target by 100%, the decrease effect on the other enemies is amplified by 100% [6];
  
• The move has a 75% chance of destabilizing the target [6].

Rain of Blades [6]
Styx throws a knife at each enemy within a wide range.

• Concentration cost: 35.
  
• Area: 8 meters.
  
• Damage: 200%.
  
• Type of impact: Light.

Upgrades:

• Every enemy hit has a 35% chance of bleeding [9];
  
• Every hit enemy has a 35% chance of suffering a vulnerability [9].

Stunning Bomb [10]
Throws a mixture of Amber that explodes and stuns the target.

• Concentration cost: 35.
  
• Damage: 150%.
  
• Type of impact: Medium.
  
• Chance of stunning: 100%.

Upgrades:

• The attack also has a 100% chance of causing a vulnerability [12];
  
• The explosion has a 50% chance of stunning any enemy withing a 5 meter radius of the target [12].

Uses his daggers to launch a fast series of hand-to-hand attacks.
Upgrades:

• Every critical hit has a 50% chance of causing a vulnerability [3];
  
• Increases Styx's concentration regeneration by 50% in Hand-to-hand stance [3].

Quick Attack
Default attack. Automatically executed when Styx fights in Hand-to-hand stance.

• Damage: 75%.
  
• Type of impact: Light.

Upgrades:

• Every hit has a 25% chance of extending the target's vulnerability [3];
  
• Every critical hit has a 50% chance of causing bleeding [3].

Flying Daggers
Styx whirls his daggers around his target, inflicting significant damage over a period of time.

• Concentration cost: 40.
  
• Damage: 50%.
  
• Number of blows: 6.
  
• Type of impact: Medium.
  
• Enemy parry: +50%.
  
• Enemy dodge: -50%.

Upgrades:

• Damage increased by 20% [5];
  
• Every critical hit has a 10% chance of stunning the target [5].

Armor Piercing
Damages a target's armor, causing vulnerability and heavy damage.

• Concentration cost: 25.
  
• Damage: 150%.
  
• Type of impact: Medium.
  
• Duration: 20 sec.
  
• Enemy armor: -75%.

Upgrades:

• If the blow is parried, the target has a 100% chance of being destabilized [5];
  
• The attack has a 50% chance of causing bleeding [5].

Destabilizing attack [2]
Inflicts moderate damage and destabilizes the target, preventing him from parrying or dodging for a long period of time.

• Concentration cost: 15.
  
• Damage: 100%.
  
• Type of impact: Light.
  
• Duration: 10 sec.
  
• Enemy parry: -100%.
  
• Enemy dodge: -100%.

Upgrades:

• Damage is increased by 100% [6];
  
• The targets chances of parrying or dodging the attack reduced by 80% [6].

Conversion [2]
Styx uses part of his concentration to regenerate his health.

• Duration: 3 sec.
  
• Life per sec.: 10.
  
• Concentration per sec.: -20.

Upgrades:

• The concentration cost decreases 50% [6];
  
• Life regeneration is increased by 100% [6].

Poisoned Daggers [6]
The next 3 hand-to-hand attacks poison the enemies, who take the damage for a long period of time.

• Number of charges: 3.
  
• Duration: 15 sec.
  
• Damage per sec.: 6.

Upgrades:

• The poison causes another 3 points of damage every sec. [9];
  
• Besides damage, poison also causes a vulnerability [9].

Focus [10]
Styx concentrates in order to temporarily increase his chances of landing critical hits.

• Concentration cost: 20.
  
• Duration: 30 sec.
  
• Critical hits: +50%.

Upgrades:

• Also increases the chances of dodging by 50% [13];
  
• Increases the effect duration to 60 sec.

Throw Goblin
Arkail throws Styx at a distant enemy, inflicting heavy damage and stunning the target.

• Damage: 500%.
  
• Type of impact: Heavy.
  
• Chance of stunning: 100%.

Upgrades:

• The attack has a 50% chance of destabilizing any enemy within a 5 meter range [5];
  
• The attack has a 25% chance of stunning any enemy within a 5 meter range [5].

Raise ally (Arkail)
Brings Styx back to his feet, restoring some health.

• Restored Life points: 15%.

Upgrades:

• Restores 50 more Life points to his companion [5];
  
• Increases chances of deflecting by 50% for all the duration of the action [5].

Small Pick-me-up [6]
Styx throws a home-made mixture that speeds up Arkail's health regeneration.

• Duration: 15 sec.
  
• Life per sec.: 5.

Upgrades:

• Also increases Rage by 5 per sec. [9];
  
• Increases Life regeneration by 100% by sec. [9].

Raise ally (Styx)
Resuscitates Arkail, restoring some health.

• Restore Life points: 15%.

Upgrades:

• Restores 100 more Life points to his companion [5];
  
• Increases the chance of dodging by 50% for the duration of the action [5].

Stealth stance
Becomes almost invisible and can fatally backstab unaware enemies.
Upgrades:

• Lets the character move faster while in Stealth stance.
  
• Allows the character to get even closer to the enemy without being detected.

Arkail's Chosen Paths

Animal Cry
A howl of Rage that increases Arkail's Rage, keeping him in Berserk state. Randomly executed when Arkail fights in Berserk state.

• Chance of launching: 15%.
  
• Rage: +50.

Upgrades:

• The Animal Cry restores 50 more Rage points.
  
• The Animal Cry restores 100 Life points.

Berserk Attack
Default Berserker attack. Automatically executed when Arkail fights in the Berserk state.

• Damage: 100%
  
• Type of impact: Medium.
  
• Critical hits: +50%.

Upgrades:

• Every successful hit has a 20% chance of causing bleeding.
  
• Every successful hit has a 20% chance of destabilizing the target.

Hurricane of Rage
Long series of area attacks that are both chaotic and devastating. Randomly executed when Arkail fights in the Berserk state.

• Chance of launching: 15%.
  
• Area: 4 meters.
  
• Damage: 100%.
  
• Number of blows: 6.
  
• Type of impact: Heavy.
  
• Enemy dodge: +50%.

Upgrades:

• The attack becomes impossible to interrupt.
  
• The combo restores both 100 Life points and 50 Rage points.

Battlement
Arkail suppresses part of his Rage to increase his chances of deflecting enemy attacks.

• Rage cost: 35.
  
• Duration: 30 sec.
  
• Deflection: +20%.

Upgrades:

• Effect duration increases to 60 sec.
  
• Deflection bonus increases to + 30%.

Rage to Win
Arkail suppresses part of his Rage to concentrate on the battle, gaining a damage bonus.

• Rage cost: 35.
  
• Duration: 30 sec.
  
• Damage: +20%.

Upgrades:

• Effect duration increases to 60 sec.
  
• Damage bonus increases to +30%.

Rage to Live
A war cry that converts some of Arkail's rage into health.

• Duration: 3 sec.
  
• Life per sec.: 20.
  
• Rage per sec.: -10.

Upgrades:

• The action cannot be interrupted.
  
• Restores 10 more Life points for the same amount of rage.

Styx's Chosen Paths

Fury
Series of 8 lighting attacks impossible to interrupt.

• Concentration cost: 50.
  
• Damage: 50%.
  
• Number of blows: 8.
  
• Type of impact: Light.
  
• Enemy parry: -25%.
  
• Enemy dodge: -25%.

Upgrades:

• Damage is increased by 50%;
  
• Every hit has a 10% chance of causing a vulnerability.

Laceration
Quick attack, a signature skill of Master Assassin.

• Damage: 100%.
  
• Type of impact: Light.

Upgrades:

• Damage is increased by 50%;
  
• Target's chances of parrying or dodging the attack are reduced by 50%.

Subterfuge
Makes Styx disappear in a cloud smoke, nullifying the threat generated on the enemy.

• Concentration cost: 80.
  
• Area: 10 meters.
  
• Threat: -100.

Upgrades:

• Concentration cost decreases by 50%;
  
• Also restores 30 Life points.

Explosion
Powerful magic explosion inflicting area damage depending on Styx's spirit level.

• Concentration cost: 65.
  
• Area: 8 meters.
  
• Magical damage: 200.

Upgrades:

• Damage is increased by 50%;
  
• The explosion has a 50% chance of destabilizing enemies.

Shadow Combustion
Ranged attack causing bleeding and damage according to Styx's Spirit level.

• Magical damage: 200.
  
• Duration: 5 sec.
  
• Damage per sec: 6.

Upgrades:

• Damage per sec. increases by 100%;
  
• Combustion spreads and injures foes within a 2 meter radius around the target.

Life Drain
Absorbs the target's life, causing damage while regenerating Styx's health.

• Concentration cost: 35.
  
• Duration: 2 sec.
  
• Life per second: 10.
  
• Damage per second: 20.

Upgrades:

• Damage per second increases by 100%;
  
• Life regeneration per second increases by 100%.

Arkail

Offensive stance is not very useful on Hard and Extreme, although sometimes using offensive skills can be the only way to win a fight in the beginning of the game - at least until the second chapter.
Both upgrades are not particularly useful, choose whatever you like.

Fast Attack - nice skill, upgrades - same as above, both are not that useful.

Powerful Blow - very nice skill with great damage. 'The powerful Blow is impossible to interrupt' is better. And overall, any upgrade with 'impossible to interrupt' quality is better its counterpart imho.

Feint - may be I don't understand how to use it, but I didn't find it useful. Looks like waste of time and health to me.

Round Attack - great skill on Medium, pretty dangerous attack for Arkail to use on Extreme difficulty. Both upgrades are equally useful.

Sworn Enemy - I can't imagine the situation when you might need this skill.

Offensive Charge - one of many attacks with fancy animations, but very dangerous to use on higher difficulty levels. I never used it.

Murderous Rage - the surest way to kill Arkail as fast as possible on higher difficulties.

Defensive stance - the best way to fight on any difficulty. 'The chance of deflecting increases by 10% in Defensive stance' - is better than slower rage increase. Any piece of equipment or skill that increases deflection is must have on Extreme and Hard.

Quick Attack - great skill. Stunning chance is better than reducing chances for dodging and parrying. Arkail's main purpose during the battle - is to hold aggro and survive.

Stunning punch - another great skill. Unfortunately, enemies dodge it way too often, even with the upgrade which reduces chances to dodge and parry.

Rage Release - useful skill, can be used for healing (with 'Converts 100% of the suppressed rage into Life points' upgrade), although like all other forms of healing in the game, it's pretty weak.

Taunt - one of the most useful skills, along with Quick Attack. 'Extends the area of effect of the Taunt by 100%' upgrade is better imho.

Back and Force - great skill but mostly for one on one fights, like duels in chapter 1. I don't know if it's a bug or what but even with 'The attack has a 100% chance of stunning the target' upgrade some enemies just refuse to be stunned. So I don't know, may be higher damage is better in this case?

Carapace - another must have skill for any difficulty. Health regeneration can save Arkail's life if he has high enough Deflection.

Disengagement - nice skill to use against hordes of goblins, better version of Round Attack. 'Impossible to interrupt' is always better.

Styx

Range Combat stance - preferable stance for Styx. 'Every critical hit increases concentration by 10' - is much better, it allows to quickly restore concentration for more powerful attacks.

Accurate Throw - great skill, despite its light impact rate it often staggers even elite enemies which is pretty satisfying to watch. 'Each critical hit has a 50% chance of stunning the target' - perfectly complements its staggering ability.

Low Blow - another skill with beautiful animation, but not very useful. Both upgrades are equally good.

Sharpshooter - must have buff, Styx needs high critical chance for more damage. 'The chance of dodging is no longer decreased' upgrade is slightly better imho.

Beard Trim - fancy animation, useless skill. After Arkail's Taunt wears off, any enemies attacked by Styx begin to target him anyway.

Rain of Blades - another fancy and long animation, that opens Styx to attacks which can interrupt this lenghty showing-off. In the worst case scenario Styx can be dead before he finishes it. Damage is not that bad though. Use this skill with caution. 'Every enemy hit has a 35% chance of bleeding' is better, but not by much.

Stunning Bomb - nice skill when it works. With 'The explosion has a 50% chance of stunning any enemy withing a 5 meter radius of the target' upgrade it often works at least on somebody, which can be nice.

Hand-to-hand stance - this stance has some very useful skills, but on higher difficulties Styx won't survive for long in close combat, even with high chance to dodge enemy attacks. Concentration regeneration by 50% is slightly more useful.

Quick Attack - nice skill. 50% chance of bleeding on critical hit is better.

Flying Daggers - very useful skill with good damage output. Especially when used with Poisoned Daggers. Both upgrades are equally useful.

Armor Piercing - great debuff, but most of the time you won't have enough concentration to use it. Bleeding is better than occasional destabilisation.

Destabilizing attack - same as above, it's great, but there are more powerful skills to use. Increased damage is better imho.

Conversion - not as good as it looks for two reasons. First, concentration is more important than health because Styx can restore some health by just attacking the enemy (there is equipment with such properties). Second, this 'conversion' animation leaves Styx open to attacks and he may lose even more health than he tried to restore. I wouldn't bother using this skill.

Poisoned Daggers - extremely useful and powerful skill. Three more points of damage are better than vulnerability.

Focus - the best buff for Styx. Learn it as quick as possible and never forget to use it in the beginning of any fight. Increased duration is better, but increased chance to dodge is good too.

Special skills

Throw Goblin - pretty dainty way to send Styx to his doom on Extreme. But can be fun on lower difficulties.

Raise ally - you will use it a lot on higher difficulties, at least during the first half of the game. Increased chances of deflecting are better for Arkail reviving Styx, because as I've already said Styx can restore his health by attacking enemies. And 100 more life points are better for Styx reviving Arkail, because Styx already have decent enough chances to dodge attacks.

Small Pick-me-up - yet another skill with long and fancy animation. Doesn't worth it. Both Arkail and Styx will lose a lot of time and health trying to make this trick happen.

Stealth stance - already explained in the official manual. Both upgrades are good. I personally prefer the ability to get closer to enemies. Faster movement will save you some time but there are about five or may be six occasions in the game where that ability to get close to an enemy can be really useful and save you a lot of trouble.

Path of the Berserker - is not the path for Extreme difficulty ) If you know the game well enough you can make very effective Berserker build, but in that case you probably don't need a guide.
The main problem of this path is that all its skills are randomly executed and you have no control over them.

Animal Cry, Hurricane of Rage - nice skills when they are activated.

Berserk Attack - great skill for Medium difficulty, both upgrades are good.

Path of the Master - this path has different kinds of problems. Arkail simply won't have enough rage to utilize these skills.

Battlement - nice skill when Arkail has enough rage for it. Both upgrades are good if you have enough skill points to get any of them.

Rage to Win - same as above.

Rage to Live - probably the only really useful special skill for Arkail. Better version of Rage Release. Unfortunately it becomes available rather late in the game. Both upgrades are good.

Path of the Assassin - preferable path for Styx.

Fury - very powerful attack but with very high concentration cost. Can be used against the target that needs to die ASAP. Works even better with Poisoned daggers.

Laceration - good skill, but nothing special. Increased damage is better than lower chances of parrying and dodging.

Subterfuge - very high concentration cost with pretty lackluster effect. You already have Arkail with Taunt.

Path of the Shadow Goblin - fun path for Medium difficulty, completely useless on Hard and Extreme.

Explosion - high concentration cost, 'powerful magic explosion' is not that powerful and on top of that the animation for this attack is painfully long.

Shadow Combustion - pretty much the same as above. It very rarely spreads to other enemies and the damage increase is difficult to notice.

Life Drain - another painfully long animation with pretty unspectacular result. Both upgrades are equally useless.

Weapon Upgrade
Any weapon gets +5 to its damage stat, any other properties don't change.

Armour Upgrade
Chestpieces get +6% to DR, any other properties don't change.
All other armour pieces get +2% to DR.

Entrance to the shantytown

Studded leather gauntlets [DR +2%, Critical hit +5%] in a sack on a cart.

Yohimbe bark [Stamina +4] in a sack on a cart.

Light greaves [DR +3%, Dodge +5%] in a sack.

Sacrificial daggers* in a chest. You can get two sets of them if Styx started the game with these daggers.

Orc Sorok drops Blood splattering cudgel [D +5, Striking force +3, Deflection +5%].

Meet the first merchant - Spearhead, he will give you Seawater Crystal.

Spearhead's goods
For Arkail:

• War Axe [D +7, Critical hit +5%],
  
• Studded leather greaves [DR +4%, Agility +2],
  
• Arms of Mother Earth [Agility +4],
  
• Breath of Mother Earth [Stamina +4].

For Styx:

• Throat Slitters [D +3, Dodge +5%],
  
• Shadow Blades [D +5, Dodge +10%],
  
• Light Armor [DR +8%, Stamina +4],
  
• Warmwood Stick [Strength +4],
  
• Phial of Brandy [Life reg/s +1].

Spearhead gives one trade point after dealing with the Scourge and another one for killing Dorek. You can challenge Spearhead stright away after the first conversation with him, even on Extreme, but don't forget to upgrade Blood spluttering cudgel first.

Reward for challenging Spearhead - Gladiator Breastplate [DR +8%, Strengh +4].

Don't challenge Fox until you finish the very first map or you will fail miserably. She dodges everything like crazy and deals a lot of damage.

The Gem district

Light Gauntlets [DR +3%, Stability +5] in the chest near Jared.
Troubleshooters [D +6, Concentration reg./s +1] - reward for killing Jared (or Oboth).
Champion Gauntlets [DR +4%, Stamina +2] in a chest.
Scarlets Boots [DR +3%, Dodge +10] in a sack.

Reward for challenging Fox - Gladiator Gauntlets [DR +3%, Critical hit +3%].

Shantytown: Scourge area

Red Breath Emblem [Strength +4] in a chest.

Phial of Human Blood [D +5] - reward for dealing with Scourge from Monkey.

Reward for challenging Monkey - Lion's fang [D +8].

Shantytown: the Trainer's hideout

Studded Leather Armor [DR +6%, Stability +5, Strength +4] in a sack.

Reward for challenging Last - +1 Stamina, +1 Mind, +1 Skill point for Arkail.

The Pass

Champion's greaves [DR +6%, Stability +5] in a sack.
Scarlets gloves [DR +3%, Striking force +5]
2 sacks with 2 trade points each.

The Catacomb

Scarlets tunic [DR +4%, Dodge +10%, Concentration reg./s +1] in a sack.
Dried mushroom powder [Agility +4] in a chest.

The Catacomb: the Shaman's lair

Crazy Shaman drops Ritual Copesh [D +9].

The Pass sewers

+1 Mind point to Styx for examining the book on a table.

Champion's Breastplate [DR +10%, Deflection +5%, Stability +5] in a sack.
Blinding stone [Dodge +10%] in a sack.

After the final battle:
Double Pain [D +8, Critical hit +3%],
Shears [D +5, Critical hit +3].


Bone Stripper [D +7, Deflection +5%] - reward for finishing 'The Miners' revolt'.

Meet your second merchant Adek.

Adek's goods
For Arkail:

• Skullbreaker [D +7],
  
• Thousand sufferings [D +8, Striking force +3],
  
• Rain of the fearless crystal [Deflection +10%].

For Styx:

• Disembowelers [D +6, Dodge +5%],
  
• Small Amber charm [Mind +4],
  
• Fermented barley ear [Striking force +5].

Adek gives Styx +1 skill point for clearing the tower after Head Foreman killed. [Only on Extreme maybe?]
Vampire Fangs [D +7, Converts 5% of inflicted D into Life points, Critical hit +3%] - reward for clearing the tower.

Mire: the foremen's quarters

Assassin gloves [DR +3%, Strength +2] in a sack.
Sacred relic grieves [DR +3%, Mind +2] in a chest.

Small bag of Holy Soil [DR +10%] - reward for killing Head Foreman. Don't forget to talk to Mahrlok to get it.

Mire: the prison wardens' refuge

Ritual gloves [DR +4%, Mind +2] in a chest.

Mire: the swamp

Sacred relic's belt [DR +8%, Life reg/s +0,5, Rage reduction/s +0,5] in a sack.

The Inquisition's Tower: Pillar

Berserker's carapace [DR +2%, D +3] in a sack.
Assassin jerkin [DR +8%, Critical +5%, Concentration reg/s +1] in a chest.

The Inquisition's Sanctuary map

Berserker's greaves [DR +2%, Striking force +3] in a chest.
Assassin boots [DR +3%, Agility +2] in a chest.

Fanged mace [D +7, Striking force +5] after the final battle.


Meet your third merchant Ered, he gives you one trade point. If you think you will be able to trade with him after dealing with Royal officer, you are wrong. This is your only chance to trade with him, use it wisely. Once you exit the dialogue window, his inventory is lost for you until the last chapter.

Ered's goods

For Arkail:

• Punishment [D +8, Striking force +5, Critical hit +3%],
  
• Sacred relic gauntlets [DR +3%, Strengh +2],
  
• Regeneration talisman [Life reg/s +1].

For Styx:

• Flotsam and Jetsam [D +4, Striking force +3],
  
• Ritual trousers [DR +4%, Dodge +5%, Stability +3],
  
• Meditation stone [Concentration reg/s +1].

Styx gets +2 Mind for choosing Path of the Assassin or +2 Agility for choosing Path of the Shadow Goblin

Arkail gets +2 Mind, +1 Stamina for choosing Path of the Master or +2 Strength for choosing Path of the Berserker.

Escaping from the monastery

Chest with 2 trade points.

Mercenary chief drops Executioner [D +12].

Gorgosh drops Berserker's straps [DR +6%, Reflects 20% of damage taken, Strength +2].


Mical can upgrade equipment, but he's not a merchant.

The Island of Laments

Ritual robe [DR +10%, Life reg/s +1, Stability +3] in a chest.
Sisters of Mercy [D +10, Critical hit +5%] in a chest.


Meet your last merchants Ered and Adek.
Their goods

For Arkail:

• Bloodjaw blade [D +6, Deflection +10%],
  
• Punishment*,
  
• Executioner*,
  
• Sacred relic gauntlets*,
  
• Hawk eye [Critical hit +5%],
  
• Bear claw [Striking force +5],
  
• Regeneration talisman*.

For Styx:

• Flotsam and Jetsam*,
  
• Slice and Dice [D +5, Striking force +5],
  
• Master Assassin's daggers [D +8, Critical hit +5%],
  
• Ritual trousers*,
  
• Ginger root [Critical hit +5%],
  
• Pain relief talisman [Stability +5],
  
• Meditation stone*.

Arkail needs armour with high DR and deflection bonuses. Best armour for him: Gladiator set < Champion set, and Small bag of Holy Soil.
Best weapons: Blood splattering cudgel < Bone Stripper < Bloodjaw blade.

Styx needs equipment with bonuses to dodge and health restoration. Best armour for Styx: Scarlets set < Ritual set, and Blinding stone. Assassin's set is good too, but I think Ritual set is still better.
Best weapons: Sacrificial daggers (can be his starting equipment) < Troubleshooters < Vampire Fangs < Master Assassin's daggers.

The main attribute for Arkail is Stamina, try to get it to 30. Styx is a damage dealer, he needs high damage, high critical chance and high chance to dodge. More than anything else he needs Strength, second important attributes - are Agility and Mind.
My builds (without any equipment bonuses) before the final fight in the Throne room look like this.
Arkail:

• Strength 16
  
• Agility 10
  
• Stamina 28
  
• Mind 11

Styx:

• Strength 23
  
• Agility 17
  
• Stamina 6
  
• Mind 18

Arkail wears Champion set (+2 Stamina from gauntlets), Small bag of Holy Soil and wields Bloodjaw blade.
Styx wears Scarlet set, Blinding stone and wields Master Assasin daggers.
Here is their overall stats.
Arkail:

• Damage reduction 40
  
• Damage 27
  
• Striking force 16
  
• Chance to critical hit 10
  
• Stability 40
  
• Life points 600
  
• Deflection 25,5
  
• Rage points 220

Styx:

• Damage reduction 20
  
• Damage 36
  
• Striking force 28
  
• Chance to critical hit 22
  
• Stability 6
  
• Life points 120
  
• Dodge 47,5
  
• Concentration per sec. 5,5

Just about before the assault on The Inquisition's Tower you will run out of useful skills for Arkail to obtain. Try to save three skill points for special skills which you will get in chapter three. Styx will always have a useful skill or two to obtain, but you will still need at least two available skill points to get Fury and its upgrade in the end of chapter three.