Necromancer

Necromancers are undead enemies. They wander around the room aimlessly. If an enemy in the room is killed, the Necromancer may raise his arms and resurrect that enemy back into battle. If no enemy besides Necromancers and Exorcists died in the room, they will be resurrected as Bonies instead.

Exorcist
Exorcists behave similarly to Necromancers but exclusively spawn ghost enemies (with the exceptions of Red Ghosts and Nightwatches) regardless of any enemies being killed. In the boss fight against The Heretic, 5 Exorcists are seen summoning him before turning into Fanatics.

Trivia

 * Necromancers' appearance bear a similarity with The Haunt, pulling their facial skin behind them and revealing their skulls through their mouths. This resemblance could have something to do with their necromantic abilities.
 * Exorcists, however, appear to have ripped their skin starting from the top of their heads.
 * The way Necromancers revive enemies is similar to the Arch-Vile from Doom, however, this may be a coincidence