Acererak

Acererak is the main antagonist of the Tomb of Annihilation campaign, whom is responsible for the death curse afflicting the world and over time aligns the events of the story to play out so that it wins in his favor, such as giving Jaffie the Deck of Many Things in disguise as an old man and ultimately causing the Destruction of Port Nyanzaru by placing the demon Barbara (possibly a doppelgänger) as the inn keeper of 'The Thundering Lizard', thus her summoning the balor.
He revealed his true form during the final events of the epilogue session, where he almost won in defeating the hero's and the Comrades, before the Lord KB and his Boomnades, followed by their newly crowned pope UwU joined the battle, thus forcing the lich to teleport and escape.