Madness can take many forms, but none so contemptible as man's belief in a mythology of his own making. A world view buttressed by dogmatic desperation invariably leads to single-minded fanaticism, and a need to do terrible things in the name of righteousness. This man is an animal - rabid, destructive, and incapable of nuanced understanding. He. must. be. put. down.
The Fanatic is a wandering boss, like The Collector and Shambler. He will not appear in the game until the Infestation Level reaches medium or high, at which point a town event occurs announcing his arrival.
Once this event is triggered, he will randomly spawn somewhere inside a dungeon whenever you bring a party with two or more members afflicted by the Crimson Curse in it. If you have 2 or 3 heroes infected, the chance of him appearing is 20% and 33% at medium and high infestation, respectively. If all four of your heroes are infected, there is a 75% chance at both medium and high infestation. If the game decides that the Fanatic is to appear during an expedition, a special loading screen with his face will show when the Dungeon is loading, and the Ancestor will also give a narration to the Fanatic's background. This is your primary warning that he is lurking somewhere within.
The Fanatic will not spawn in boss quests due to their loading screen having priority over his. This makes bringing any amount of cursed heroes to these quests safe. You are also spared from facing the Fanatic if you're venturing into the Courtyard or the titular Darkest Dungeon. If you complete an Epic Courtyard quest, which removes the Crimson Curse from your entire roster and resets the Infestation Level, you will not encounter the Fanatic until the Infestation Level builds back up again. If you have slain the Countess, the Infestation Level disappears, and you will never encounter the Fanatic again for that playthrough.
Behavior
Upon entering battle, the Fanatic will always surprise the party. If you decide to run away from the Fanatic, he will appear in the next encounter, fulfilling his promise to hunt "the Cursed" down. He will spawn in the two back ranks with his stake and pyre in the front two rows. He has three actions per round, and attacks with various skills that can cause debuffs, bleed, stuns, and horrors, on top of moving him around the battlefield.
The most dangerous part of the fight is his "Sentence Rendered" skill, which, if not dodged, binds a hero to his pyre and causes them to take damage over time. Similarly to the Hag, a hero currently burning at the stake will not retreat with the rest of the party if the party retreats from combat, but will instead count as having died. In addition, if this hero had any trinkets on them when you retreated from combat, those trinkets will be lost. Once the hero reaches Death's Door, they are released from the pyre to the front of the formation.
Unlike the Hag's cauldron, the Fanatic's pyre does not restore its durability after a hero gets released from it. If the pyre is destroyed, the Fanatic will no longer be able to use "Sentence Rendered" to deny use of your heroes, and instead start using "Fury of the Righteous" as his first move on every round, dealing physical and stress damage to your entire team in order to avenge his pyre.
Drops Crimson Curse-related trinkets, such as the Ancestor's Vintage and the Coven Signet. Subsequent Fanatic defeats will yield gem loot and other trinkets related to the Crimson Curse. Additionally, he can drop a special item known as The Cure. Any hero who uses it will be immediately healed from The Crimson Curse. Like The Blood, all unused vials of The Cure will be stored upon returning to the Hamlet.
Its origin and ingredients unknown, but this tincture will quell the Curse.
Strategy
The Fanatic is a relentless boss that you may not be prepared for when he decides to attack. He behaves like a mixture of the Hag and the Shambler, as he has a pyre that occupies the other two ranks of his side. Unlike the Hag, he moves around a lot so close-range heroes have opportunities to hit him. However, like the Shambler, he will always ambush the party when he appears, which can throw unprepared parties into shambles. If you attempt to retreat he will chase down the party and appear as the next encounter. At that point, the only way to truly escape from him is to abandon the quest and return to town.
At 3 turns per round, DoTs - Bleed, especially - will progress on him very quickly. Since he has 15% prot, bleeding him out is probably going to be the best strategy. If you prefer direct strikes, consider the Grave Robber or Shieldbreaker to pierce his prot. Riposte from the Highwayman or Man-at-Arms is also quite useful, due to his numerous turns. What's not as effective are debuffs and stuns, since his numerous turns will quickly drop them off.
The Fanatic will eventually grab a hero to burn at his stake. They will burn until they reach 0 HP, then come back at death's door. You could destroy the stake to stop this. If you do however, the Fanatic will use a new skill every round that targets the entire party for damage+stress. You should decide if you want to leave the stake, or destroy it. If you leave it, be prepared to heal heroes when they come back. If you want to destroy it, consider buffing defenses and virtue chances on the party. Sentence Rendered can be dodged, at which point the Fanatic will only use his other abilities for the rest of that round and will not attempt to use Sentence Rendered until the next round, giving you an additional action with a character.
Sometimes, discretion is the better part of valor, and the best option is to not encounter the Fanatic at all by properly managing your Cursed heroes. If he's found you while you're not prepared, you may want to cut your losses and abandon the quest.
Team Selection
Recommended Heroes
Grave Robber, Shieldbreaker - Both have movement skills to counter the opening ambush, and PROT-piercing attacks to push past his 15% PROT.
Highwayman, Man-at-Arms - Riposte can easily counter his many actions per turn. If you've destroyed the pyre, they can punish his area-hitting attack.
Vestal - Make sure that your team can compensate for her lack of mobility to put her in an optimal position on the first turn.
Flagellant, Hellion - Can inflict Bleed to him regardless of where he stands.
Bounty Hunter - Damage bonus vs Humans greatly aids the fight. The Fanatic moves into range of Collect Bounty frequently. If you can predict when he shows up, you can camp and use Planned Takedown to increase damage.
Reasonable Choices
Crusader - Holy Lance to force him out of the back in the initial shuffle. Can support from the front lines with Battle Heal and Inspiring Cry, though they are minor boons.
Leper - Delivers high raw damage to quickly cut down the Fanatic or smash apart the pyre. Struggles with the initial shuffle.
Antiquarian - Largely unaffected by the initial shuffle. Invigorating Vapours can raise DODGE to avoid getting your heroes bound to the pyre.
Discouraged Heroes
Plague Doctor - The Fanatic's three turns makes her Stuns less viable, and his Blight resistance neuters her specialization. However, with the help of some of her trinkets her Blight abilities will be an option, and the use of her Emboldening Vapours skill will not be a waste of a turn, should she end up in her unfavorable front position, or even regardless of her current placement.
Occultist - His debuffs wear off very quickly, his pulls and stuns are largely irrelevant during the fight, and a heated battle like the Fanatic's fight is not ideal for rolling with his healing.
Arbalest, Musketeer - Like debuffs, Marks are very short-lived, so you have little opportunity to strike the Fanatic for increased damage. They're great if you want to destroy his pyre quickly, but suffer a lot if they're the ones on the pyre.
Houndmaster - The Bleeds he inflicts are really minor, and his stuns get shaken off very quickly.
The Fanatic is voiced by Youtuber/Streamer BaerTaffy, whose longest and most popular series consist of Darkest Dungeon playthroughs, going well over 400 episodes and hundreds of hours of playtime. Baertaffy is also the voice of The Viscount and The Baron.
The Fanatic wears multiple Trinkets on his person, including a Book of Holiness, Fortifying Garlic, Glittering Spaulders, Holy Orders, and several Martyr's Seals.