Orcus

Orcus is a demon lord and master of the undead.

Appearance
Orcus was typically described as having the head and legs of a goat, although with ram-like horns, a bloated body, bat-like wings, and a long tail.

Background
Not very much is known about Orcus besides that he is the master of the Undead. He has armies of fiends, devils, demons and the sorts under his command working on a way to get Orcus out of the plane he is on and make his way onto the prime material plane.

"Yesterhearth" (1x03)
Orcus was seen in a vision by Ashtaranu when he touched The Chromasphere. Orcus then marked Ash with the Mark of Orcus, making him a target for all his minions.