The Dark Reaper
Orcus Nerull, The Dark Reaper, is the Chaotic Evil God of death, slaughter, and necromancy. He was one of the Seven Creators, and was the god of death, the underworld, and crafting until he fell to evil. He was originally known as The Keeper. He created the dwarves as servants in the underworld, where they shaped the souls those who died into new forms.
During the First War He had the dwarves fashion the souls of the dead into weapons, and then grew mad with the power over death. He began to horde souls in the underworld, keeping them for power at first, and then simply keeping them because he felt they all belonged to him. The dwarves were enslaved in the Shadowfell until they rebelled, much like the eladrin did in the Feywild. They forged chains to bind him, but they could not hold him until The Raven Lord came to their aid, leading them out of The Shadowfell into the World. The Dark Reaper broke those chains, and attacked the world, but The Raven Lord fought him.
Orcus was killed in this battle, but his spirit possessed the Raven Lord and manipulated him. For millennia, The Raven Lord was regarded as little better than the god he had replaced. Finally, The Raven Lord created an avatar from the bones of Orcus to serve as an Exarch. Orcus’s spirit took the opportunity to possess the construct and went on a rampage known as The Grim Harvest. The Dark Reaper was eventually defeated, but the gods decided to cast him into the Frozen Hell rather than risk his spirit being free.
During the battle with the gods, Orcus cast two artifacts into the world to serve as anchor points that could be used to free him. one was his scythe Godslayer, which was eventually recovered by servants of The Raven Lord. The other was his black, undead heart which has so far proved elusive. The Witch Lord is said to have found the heart and used it to create his undead horde during The Hundred Years Chaos, but only The West Star Companions would know for sure.