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Mephistopheles
Mephistopheles is the archdevil ruler of 
Cania, the eighth layer of the Nine Hells.
Overview
While he himself does not appear within Baldur's Gate 3, he has a presence within Astarion's Origin quest. Mephistopheles tieflings descend from bloodlines affected by Mephistopheles.
Mephistopheles specialises in deals involving magic, which is made evident in his deal with Cazador, which involves a ritual so dastardly and foul, none have attempted it - one that will create the Vampire Ascendant.
History
After the fall of Netheril, Mephistopheles sequestered the Crown of Karsus away in one of his archival vaults. Mephistopheles' son, Raphael, coveted the crown, but was unable to obtain it for a thousand years. Eventually, the crown was stolen by the cult of the Absolute, and Raphael put into motion plans to take it for himself.
The incubus Haarlep was also sent by Mephistopheles to distract his "naughty son", possibly from his ambitions for the crown.
