Cultist

Cultists are an enemy added in The Binding of Isaac: Repentance.

At the start of the room, they create a homing, indestructible Purple Balls (like Rag Mega's) and proceed to follow it around on a brief delay. These purple balls fade into Dark Balls after 6 seconds, which can be destroyed as normal and have 10 HP. They cannot spawn another ball as long as the one they previously spawned is still intact.

If they detect an enemy has died in the current room, it will stop following the ball it summoned, move to where the enemy died, and resurrect it with full health before going back to following its ball. Cultists cannot revive other Cultists.

Blood Cultist
Blood Cultists are a variant of Cultists, but are similar only in appearance and their mobility. They wander erratically around the room and, if Isaac aligns with them horizontally, they will fire a horizontal wave of red creep and blood shots with unlimited range. They can also summon four bone traps that prevent Isaac from moving for one second when stepped on, predicting where he will be when summoning them. The bone traps cannot be destroyed in any ways other than stepping on them, and de-spawn after 3 seconds if not stepped on.

Trivia

 * Cultist might be a reference to the Floater enemy from The Legend of Bumbo, despite there actually being a Cultist enemy in the game.