Necragmancer is an enemy added in Revelations Chapter Two.
Necragmancers stay wrapped in bandages and are invincible until a buffable enemy dies. He will then unwrap himself, becoming vulnerable, and attempt to revive the enemy by sending a large tear towards the corpse of the dead enemy. If Necragmancer revives all possible corpses in the room, he will wrap himself back up in bandages. This will repeat until Necragmancer dies or all buffable enemies die after being buffed. While Necragmancers can be killed, it isn't required to clear the room.
If killed after the Pact of Revival was picked, Necragmancers will drop red health, ranging from half a heart to a full heart.