In case you're reading this and somehow don't know... to unlock "In the Mouth of Madness" you have to beat the Sleeper, the boss at the end of wave 3 within the Farmstead, with 2 afflicted heroes and 2 virtuous heroes. Not an easy task! Many achievements in the Darkest Dungeon hang on luck, but this one is riddled with factors that largely depend on RNG. You have to fight your way through 3 full waves of enemies (including the Miller boss fight), have your heroes resolve properly, and then kill the Sleeper without losing any of them. Yuck.

A few things to note:

• when stress reaches 100, every hero has a 25% chance of becoming virtuous, and a 75% chance of becoming afflicted (save the Flagellant, he will always become afflicted)
  
• in endless mode, characters will hang on to their virtuous/afflicted status - to prevent meddling with your battles, it is best to try and keep the 2 heroes you want afflicted from stressing out until you're ready to fight the Sleeper
  
• the negative quirk Mercurial takes away 5% virtue chance, while the positive quirk Irrepressible gives you 5%
  
• Shard Dust can be purchased before entering the Farmstead, and it gives your character -10% virtue chance, while giving a nice damage boost as well - use it on the appropriate heroes
  
• the Blood can be used on a non-Crimson Curse carrying hero to give them 25+ stress, at the cost of some health - bring some along just to be safe
  
• passing a turn adds 7-ish stress to the hero

So the first (almost mandatory) choice is the Highwayman. With both of his Crimson Court trinkets, his virtue chance is boosted to 70%, while still granting him some decent speed, dodge, accuracy and crit. Fighting your way to the Sleeper is half the battle, if not more, and he carries himself well, especially with his riposte.

Next, a Jester. Being able to control the stress levels of your heroes is key, plus his Battle Ballad is an excellent ability to spam nearly every turn you're not controlling stress. I gave him the Tentacle Idol and the Hero's Ring, giving him a 70% virtue chance as well. These trinkets leave your character lacking in stats, but the Jester isn't here for combat.

For the healer, a Vestal. With the Atonement Beads, it bumps her virtue chance down by 15%. They don't offer anything else unfortunately, but with a healing trinket in the other slot, she still does her job well enough; I used the Ancestor's Scroll. Her stun was also clutch throughout the whole run up till the Sleeper - the majority of enemies have 60% stun resist, giving you an 80% chance to knock them out for a round. Even on the big boys with 90% stun resist, a 50% shot at stopping them is worth it when your team is healthy.

You may be tempted to use a Flagellant as a healer, considering he is guaranteed to become afflicted. However, I have had poor success using him in the endless mode, he seems somewhat unreliable and his big heal can only be used twice per round. Still, a tempting option if you can make it work; the less randomness you have to rely on, the better.

To round out the team, I went for a Leper. He's a big hitter, and you'll be grateful for his one-hit slaying power, both during the waves and during the Sleeper fight to finish off the boss quickly. I gave him the Lens of the Comet to knock his virtue chance down by 20%, and the extra crit is perfect (as is one-shotting stealthed enemies). I used the Ancestor's Pen in the other slot, crit and damage are what you're bringing him along for - with the Battle Ballad constantly stacking, he gets a heavy crit chance and really pulls the team along. Note: you could also use the Berserk Mask with the Leper (-10% virtue chance), but it was one of the few trinkets I didn't have.

The general strategy for pushing through the waves is simple enough. Keep Battle Ballad stacked as much as possible, and watch your team's stress. Keep the Highwayman's riposte up, alternating between Duelist's Advance, Point Blank Shot, and picking off enemies in the backline. The Leper needs to smash the front line enemies, no surprise there. Be thoughtful though - don't waste a Hew unless you're pretty positive you can kill at least one of them, and line it up so you can hit a stealthed enemy (if you didn't take the Lens). The Vestal's moves should be self-explanatory, but at times it is better to take a risk and go for the stun, sometimes even moreso when you're in a dangerous scenario... that is where personal discernment will come into play to know what will give you the greatest chance of survival.

There is obviously RNG that determines what types of enemies you'll face, nothing you can do about that. Some will be harder than others, particularly if powerful enemies spawn in the back ranks (Swine Skiver, Hateful Virago, etc). In those scenarios, the Vestal's stun saved the day for me.

When you get a camping opportunity, try to buff the Leper first and foremost - Tiger's Eye and Reflection are must casts. If you don't need to heal/destress, more strong options are Unparalled Finesse and Clean Guns for the Highwayman to give him some extra killing power. If you have the Outsiders Bonfire district building, you can cast all 4 of these in one sitting. There's no need to prevent ambushes, they can't trigger in endless mode.

The Miller can be difficult if he casts the Reaping over and over, again more RNG to deal with. If he doesn't spam it, it shouldn't be too hard to knock him out if you've kept your buffs up. The Leper will pull you and the Highwayman (with plenty of riposte crits) will carry his weight as well.

Now, as far as stress management: through the first two waves, I kept everyone's stress close to zero. Entering wave 3, I switched to focusing on keeping the 2 afflicted heroes from accumulating too much stress, while ignoring the 2 virtuous heroes stress levels. Unfortunately, RNG comes in once again when it comes to your virtuous heroes, as the highest you raise your chances normally is 70-75%. However, if you happen to enter a Splendorous refraction, take advantage of the bonus 15% virtue chance (bringing your favorable odds up to 85-90%) and get those 2 stressed out. There's already so many factors to contend with, the more reliability you can add the better.

I actually found I did a little too well during this run, healed way more stress than I wanted to. Don't fret if that happens to you though, you'll be able to manipulate stress levels during the first stage of the Sleeper.

When you start the battle, the first phase is the setup. Focus on raising stress to where you want it to be - for the afflicted heroes, somewhere between 50-70% full is what I aimed for. The Fracture itself can't do much damage to you, so you can take your time for the most part and try to position yourself appropriately for the next phase. Destroy all Aberrations it summons, except the Traumatic ones - they'll stack horror on your heroes, adding stress. Pass turns when possible, use the Blood if you have a safe opportunity to heal up afterwards. At this point, all the crits may become slightly annoying, as they'll heal stress from the heroes you're trying to stack it on. It may take awhile, but I finally got myself set up decently.

Once you enter the second phase, start going to town on the Sleeper. He has a fair health pool, but the Leper should take massive chunks out of his health very quickly. Honestly, you should be able to just wreck him at this point, with all the buffs your team has compiled, but again: management. Bring him down to a point where you feel you can burst him in a couple turns - his attacks deal a ton of damage and will stack on the stress rapidly, and once your 2 afflicted heroes resolve you'll want to finish him as soon as possible to avoid any mishaps. Remember, the Highwayman's riposte will trigger on both of the boss's attacks (and usually crit), so keep that in mind when planning out your burst damage. Let the Sleeper have it and get that achievement!

I had tried different team combinations before this one but with no success. However, these things I theorycrafted helped me out tremendously and I unlocked the achievement the first run I did with this particular group and gameplan. As I said, there's a lot of RNG involved in this process. So if one of the factors doesn't line up for you, you'll have to start again... but it's only a matter of time before the stars align and victory is yours!

If you have any questions or suggestions, comments on what you did for the achievement, improvements I could add into this thing, or if any information has become out of date, feel free to leave them below! Good luck out there, and be wary - triumphant pride precipitates a dizzying fall.