The Heretic will always have an arena filled with extinguished campfires, which he uses for one of his attacks.
At the beginning of the fight, five Fanatics will be gathered around a pentagram circle. They are invulnerable until The Heretic appears. When The Heretic spawns, he will be invulnerable to damage similar to The Haunt's first phase until all the Fanatics have been killed. The Fanatics walk towards Isaac to deal contact damage, occasionally firing homing tears.
The second phase has The Heretic coming out of his invulnerability to attack Isaac. He has three attacks:
- The Heretic will point to extinguished fireplaces, lighting them up one by one as Purple Fireplaces. Once several have been lit, The Heretic will fire a brimstone beam, hitting the fireplaces that connects them in the order of lighting (Fire 1 will connect to Fire 2, Fire 2 to 3, etc.).
- The Heretic will vanish, and appear either to the left or right of the player, and will charge and attempt to bite Isaac, homing slightly. He does this four times, alternating which side he charges from. This attack deals 1 full heart of damage.
- The Heretic may change this attack slightly with a fakeout, suddenly fading out halfway through the fourth charge, and instead going for a fifth charge in the opposite direction, catching off-guard players.
- The Heretic will start lobbing purple tears at the ground that splash into a ring of six fires that spread and vanish after a certain distance. He does this several times, and ends it by moving above Isaac's position and slamming into the ground, spreading lines of purple fire in eight directions.
- Upon death, he will release lines of purple fire in the cardinal directions.
- The fires can be extinguished as he's pointing at them using bombs or other explosives, which reduces the area of the beam's coverage.
- The Heretic's general appearance as well as its damaged face suggest that it is a yet deader form of The Haunt and/or an alternative to The Forsaken. Judging by the general theme of the Mausoleum as well as the summoning animation seen at the start of the battle, it seems likely that The Heretic is a version of The Haunt that has been revived (to an extent) by necromancy.