Exorcists are enemies found exclusively in the fight against The Heretic, and can never be found naturally.
If spawned by itself, it will start to walk towards Isaac, occasionally spawning a Lil' Haunt.
After one of its Lil Haunts dies, it will leave behind a small red spirit, sometimes, an exorcist will walk over to one of the red spirits and revive it, similar to Rag Man or a Cultist.
An exorcist can spawn a max of 3 Lil' Haunts.