Orcus

Orcus is the Demon Prince of Undeath and the Lord of Necromancy. Orcus has a dedicated cult primarily composed of necromancers who do his bidding.

Description
Orcus resides in Thanatos, one of the infinite layers of the Abyss. He has an ever-hungering need to break all life so he can bring it under his power, and in Thanatos he does so. He is wholly evil and enjoys torturing and corrupting mortal souls.

Orcus is a servant of the Betrayer Gods, though Highbearer Vord identifies Orcus as one of the Betrayer Gods himself, sealed behind the Divine Gate.

History
The two Horns of Orcus, one of which was recovered by Vox Machina, are horns from Orcus' own head. In ancient times, he cut two of his four horns off, enchanted them, and placed them on the head of his "shadow champion", who was eventually defeated. The horns remained, however, and were sealed.

The Calamity
During the Calamity, the Demon Princes of the Abyss had a rare moment of unity, for the purposes of fighting the Prime Deities. The historical seat of their power on the Prime Material Plane was the Umbra Gates in Xhorhas.

Scattered War
During the iron rule of Drassig, the hierophant druids of the Dawn Circle were subjected to great cruelty and killed at one of their ritual sites, drawing Orcus's attention. He waited for centuries afterward for a mortal to disturb its deepest sanctum.

Campaign One: Vox Machina
A beholder named K'Varn discovered the hidden vault containing one of the Horns of Orcus and bent all his attention to unlocking it, which he eventually did. K'Varn affixed the Horn to himself and used that power to bring both the mind flayer colony of Yug'Voril and the duergar of the Emberhold under his sway; he compounded the threat by experimenting with the creation of stitched-together abominations. Their combined forces began to threaten Kraghammer before Vox Machina and Lady Kima of Vord slew him. Though the adventurers were unable to destroy the Horn, they ensured that it was sealed once again, this time in the Platinum Sanctuary.

After Vax'ildan became the Champion of the Raven Queen, Vex'ahlia learned from a book that Orcus seeks to claim the Raven Queen's throne and that the two gods are fierce enemies due to the Raven Queen's hatred of necromancy and undeath.

During Vax's resurrection ritual in Vesrah, Vex offered a bargain with the Raven Queen herself, saying that if she brought Vax back to life then Vox Machina would hunt down and kill Orcus for her. The Raven Queen seemed to accept, as she made an appearance before the group and told them she would call for them when the time had come, but in fact the Raven Queen didn't take that offer seriously enough to be her main motivation for helping with the ritual. Vex continued her research into Orcus and the Raven Queen during Vox Machina's long break from adventuring after defeating the Chroma Conclave.

The adventure hook Clutch of the Demon Lord in the original Tal'Dorei Campaign Setting sourcebook, set in 812 PD, involves Orcus sending a flood of undead spilling from the Shadebarrow, the site of Drassig's great cruelty to the Dawn Circle druids, requiring the heroes to take on Orcus himself.