The Heretic

The Heretic is a boss added in The Binding of Isaac: Repentance that can appear in the Mausoleum.

Behavior

During the beginning of the fight, 5 Exorcists, which are shielded and invulnerable, summon The Heretic from a large pentagram, after which they turn into Fanatics.
- The Heretic is incorporeal until all Fanatics and the Pacers or Gushers they turn into are dead, similar to The Haunt's first phase.
- While incorporeal, The Heretic can still do contact damage.

Lobs 7 projectiles that, after landing, burst into 6 fires that spread out in a radial pattern. The landing position of each projectile is marked on the ground.
- The Heretic then follows up by launching itself towards Isaac, creating a radial burst of 8 lines of fire where it lands. It spends a short time readjusting its dislocated jaw before continuing to attack.
The arena of the Heretic always spawns a number of unlit Fire Places. The Heretic can light up 5-6 of them before firing a purple Brimstone laser that connects all of the lit fires. The order the Heretic lights each fire indicates what order the laser travels.
- These can be extinguished with an explosion, preventing Brimstone lasers from reaching the deactivated Fire Place.
Dashes off-screen and turns invisible before charging in again, predicting Isaac's movements and attempting to bite down on where Isaac is moving, doing a full heart of damage if the attack connects. It chains this attack a total of 4 times, each time from alternating sides of the room.
- The Heretic occasionally does not finish the fourth charge, instead fading away and quickly charging in one final time from the other side of the screen.
Upon death, the Heretic's fiery eye drops down and sends out lines of fire in the cardinal directions.

When not fought as a main boss, such as in a Boss Challenge room or a degraded boss, it will not be preceded by Fanatics and have no first phase.
- The Heretic also cannot perform its Brimstone attack outside of its regular boss room as there will be no unlit Fire Places.
The Heretic can be instantly killed with Vade Retro if it is at 50% health or below.

The Heretic originates from a Reddit post by NotYourSaggitarius in 2015, as a versus screen mockup. The boss was later incorporated into the Antibirth mod.
It appears to be a resurrected variation of The Haunt or The Forsaken.
The only time the Heretic's empty right eye lights up is during its Brimstone attack.
The Exorcists that appear during the beginning of the fight to summon the Heretic are found nowhere else in the Mausoleum, unlike in Antibirth, where they were a regularly appearing Monster.
If The Heretic is destroyed with a Chaos Card before the summoning animation finishes, any remaining Exorcists will become vulnerable and be stuck performing the ritual until they are killed.