The Withermancer is a Dungeon Mob rarely encountered in The Catacombs - Floor IV and above. It takes 80% reduced Magic Damage, making Magic Weapons less effective against it. It also has two orbiting skulls around it that track down players that get close to it, dealing damage if the skull connects with the player.
Its ❤ Health, like any other Dungeon Mobs, scale on which floor they spawn in. It can also have modifiers such as Healthy, Flaming, Stormy, Speedy or Fortified. Healthy increases the mob's ❤ Health. Fortified increases the mob's ❈ Defense. Flaming makes melee attacks light the player on fire. Speedy makes the mob move much faster. Stormy makes the mob strike lightning on nearby players, dealing true damage.
The orbiting skulls of the Withermancer appear to reference the Bloons Tower Defense 5 Glaive lord's 4th tier upgrade on upgrade path 1 to have 2 orbiting glaives dealing immense damage to anything that touches them.
If you kill the Withermancer before it's skull hits, the skull will despawn. Additionally, skulls despawn after 10 seconds of pursuing a player.
When it is defeated, it will drop a skull stack like any other dungeon skeleton.