Orcus

Orcus, also known as the Demon Prince of Undeath or as the Blood lord, is a fiend whose power rivals that of the gods. Ruling over hordes of demons in the abyss, Orcus yearns to transform the multiverse in a gasthly place of death. Many undead creatures worship him or seek to bargain with him in exchange with some fragment of his over over undeath
While not directly participating in the blood war in Avernus, Him gaining power would also mean more power to the demon side, which is why Zariel has interest in stopping his gaining of power in the mortal realm.