Summary:
If you control exactly one demon on the board and no other units with the demon synergy, that demon’s right clicks/auto attacks do pure damage (i.e. it bypasses armour and magic resistance and it not affected by physical or magic damage amplification)
This pure damage is called ‘Fel Damage’



Conditions for Bonus:
• You must only have one demon unit on the board.
• There must be no active demon synergy from other units - a second demon will cancel the bonus for both
• Demon hunter units played by the enemy (most commonly Anti-Mage and Terrorblade, but also Vengeful Spirit and Spectre) remove the demon bonus on your board

Further details:

There are currently 7 demons in the game: Chaos Knight, Shadow Fiend, Shadow Demon, Terrorblade, Lion, Doom, Queen of Pain and Sven; Grimstroke is currently taking a break but was available as a one cost wizard until recently



Although there are demon comps (explained later), the most basic use of demons is taking advantage of their pure damage auto attacks and high damage spells as strong editions to your board in other comps. (CK in Knights, Doom in Warriors, etc. or Meepo/SF, Gods + QOP)

One friendly demon hunter: puts an invisible demon on the opponent’s board

• If they are playing no demons it gives the opponent the demon synergy
  
• If they are playing one or more demons, it cancels their demon synergy, removing the pure damage from their demon’s auto attacks
  
• If the opponent is playing a synergy that require no other synergies – usually Faceless or 4 wizards, placing an invisible demon on their board essentially breaks this unless they are already playing demons

Two friendly demon hunters: all friendly demons have Fel Power

• This means that no matter how many demons you play or demon hunters your opponents play, your demons will have pure damage auto attacks
  
• This is useful on board where you would like to play Sven and Queen of Pain at the same time (for example) or if you would like to play a full board of demons
  
• If you don’t play two demon hunters, when your Terrorblade transforms, his transformed units will lose their demon synergy and do physical damage
  
• Note: you cannot heal/lifesteal from pure damage, so the warlock bonus doesn’t work, nor do lifesteal items such as Helm of the Dominator, Satanic, etc. The only exception is CK’s heal on crit

Four demon hunters: all your damage – right-click and spells - is now pure damage

• It is useful if you have a carry that has good AOE damage (such as Spectre, Gyro, Medusa) or your opponent has a lot of physical resistance / damage block / damage return, such as 9 warriors, assault cuirass, etc..
  
• This is most common with three demon hunters and two wizards
  
• There are multiple ways to get the demon hunters not in the pool:
  
• You can get Vengeful Spirit and Spectre using Ascetics Cap (2* unit) or from a Panderan drop if you are playing undead units
  
• Spectre is available through golden rolls
  
• One of the three synergies that Ringmaster can spawn with is Demon Hunter
  
• You can find demon hunter through the use of the wheel relic

You can see what kind of damage your units are doing in the fight in the top left under ‘damage stat’ - red is physical, blue is magical, gold is pure



Additional notes on Fel Damage:

• Fel damage is amplified by the number of demons on the
  board - 15% per demon up to 7
  
• It is amplified by Sven’s ulti
  
• Since it is hard to balance, how Fel Power works changes often - it used to prevent mana generation, used to only work on the primary auto attacks of non-dem
  ons, etc.

The current most common demon build is to roll on level seven or eight for the following units:
Shadow Fiend, Terrorblade, Shadow Demon, Riki 3*
Anti Mage, Bane, Lion or Doom 2*



Shadow Fiend’s ultimate is amplified per demon on the board (up to 7), so his requiem does a huge amount of AOE which the Satyr fear from Riki and Bane guarantee. Meanwhile, the Shadow Demon amplifies the damage and Terrorblade transforms into a pure damage, right-clicking monster

For more information on this build, check out these YouTube videos by OliYoun here:

Sven is Absolutely Insane In Demons

Demonssss