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' presence in the adventures of Vox Machina is very significant as one of his artifacts, the Horn of Orcus, is obtained by the mad beholder K'Varn, who is the primary antagonist to Vox Machina in for the first portion of season 1.