Headless Destroyer

The Headless Destroyer is a powerful hostile mob that spawns on the Death Day Event. It is an upgraded version of the Headless Hunter.

Spawning
Headless Destroyers spawn in the Overworld during theDeath Day Event. They will despawn if the player gets too far away from them, or if the difficulty is set to Peaceful.

Headless Destroyers can be spawned using /summon nevermine.HeadlessDestroyer.

Drops
Headless Destroyers will occasionally drop 2-4 Copper Coins at death. This cannot be increased with the Looting enchantment.

They will also drop 35 hunter xp and about 3 XP orbs when killed.

Behavior
Headless Destroyers will pursue the player when the player gets 16 blocks or less from one. They will always float on top of water, even if their target is beneath them.

They are powerful mobs, capable of insta-killing unarmored players on Normal difficulty. They precautions when attempting to fight one.

References/Sources
http://adventofascension.gamepedia.com/Headless_Destroyer