Acererak

The Grand Architect, the Reaper of Souls, the Master of Death, the Arch Lich, Acererak the Eternal.
It is he who has seemingly constructed the Soulmonger, linked to the Death Curse afflicting Chult. He is ancient, a master of traps and schemes that lead to the ruin of many powerful heroes tempted into confronting his lairs.
His works are many, each sustained by his dark arts, magic so evil it warps reality itself. As a Lich, Acererak has artificially extended his lifespan, at the cost of his humanity, though for mages as wicked as he, it is a small price.
Many have sought out his writings and works, to understand and use for themselves, and he has taken and discarded many acolytes over the centuries, more still have been corrupted and twisted by their failure, such as M'Kufu Uwanja.
Nanny Pu'Pu knows of him, she told Tarlok how her and her Sisters initially worked with him until she broke away, hating the nature of what they were creating. She tells the story of Omu, of the Nine Trickster Gods, and how Acererak murdered them.