Triclops

The Triclops is a hostile mob that spawns on the Death Day Event.

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

Triclops can be spawned using /summon nevermine.Triclops.

Drops
Triclops drop 3 XP orbs when killed.

Behavior
Triclops will pursue the player when the player gets within 16 blocks of one. They will always float on water, even if their target is below them.

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