Orcus

Orcus is the demon lord of undeath and necromancy. He is wholly evil and enjoys torturing and corrupting mortal souls. Orcus has a dedicated cult primarily composed of necromancers who do his bidding.

Orcus's presence in the adventures of Vox Machina is very significant as one of his artifacts, the Horn of Orcus, was obtained by the mad beholder K'Varn, who opposed Vox Machina during their journey through the Underdark.

The topic of Orcus came up again 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 the two beings are fierce enemies due to demon lord's domains of necromancy and undeath.

During Vax's resurrection ritual in Vesrah, Vex directed her words to the Raven Queen herself, telling the goddess that if she brought Vax back to life then Vox Machina would hunt down and kill Orcus for her. The Raven Queen apparently accepted, as she made an appearance before the group and told them she would call for them when it was time.