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.
The two Horns of Orcus, one of which was recovered by Vox Machina, are horns from Orcus' own head.
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 apparently accepted, as she made an appearance before the group and told them she would call for them when it was time.