Glabrezu

Treachery Demon that impersonated (or possibly fabricated) the Devil Prince Juurezel. Was ostensibly the orchestrator and driving force of Demons in Emon. Attempted to deceive Vox Machina during those events, but Vox ultimately defeated him in combat. Killed Pike.
Possible that he could return with a vendetta against Vox Machina at some point in the future. I know fiends such as Hotis the Rakshasa are simply banished from the Material Plane when they are defeated and can someday return, but I'm not certain if this is exclusively a fiendish trait or if it also applies to demons.