Demogorgon

Demogorgon is an incredibly powerful Demon Prince, even claiming the title of 'Prince of Demons,' and is one of the most powerful tanar'ri. He is the Imprisoned One in Watcher's Keep. He can use up to quest-level priest spells (such as Aura of Flaming Death) and can summon demons from the Abyss to aid him. He is arguably the most powerful boss in any of the Baldur's Gate trilogy games.