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Mephistopheles
Mephistopheles is the archdevil ruler of 
Cania, the eighth layer of the Nine Hells[1]
Overview
Mephistopheles does not appear in Baldur's Gate 3, but his presence can be felt in many of the questlines and other game elements
Mephistopheles is known for his impressive magical skills. Mephistopheles Tieflings descend from bloodlines affected by the archdevil, and inherit some of his proficiency with magic.
Mephistopheles specializes in Infernal rituals, one of which was known as "The Rite of Profane Ascension". The ritual involved the sacrifice of seven thousand souls to the archdevil. In exchange, the vampire performing it would become a "Vampire Ascendant", gaining additional powers and nullifying many of the traditional vampire's weaknesses, like burning in the sun or their sanguine hunger.[2]
The vampire lord Cazador Szarr (and perhaps Vellioth before him) sought to perform this ritual, acquiring sacrifices through his vampire spawn, one of whom was Astarion. Cazador inscribed the backs of both his spawn and sacrifices with Infernal runes, necessary if the rite was to be performed.
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 over a thousand years. Eventually, the crown was stolen by the cult of the Absolute, and Raphael put into motion his 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.
