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Adventure Path Charter Subscriber; Pathfinder Rulebook, Starfinder Adventure Path, Starfinder Roleplaying Game, Starfinder Society Subscriber
In Book of the Damned Volume 2: Lords of Chaos, there are rules for a series of rituals that allow a mortal to (in order) swear their soul to a demonic patron/the Abyss, gain a demonic implant, become a half-fiend, and transform into a demon (keeping all class levels). Note that these "rituals" are actually special rites and vile actions (including evil plots and other acts that are not what many consider to be "ritual" acts) over the course of months or even years.
In Book of the Damned Volume 3: Horsemen of the Apocalypse, there are rules on binding, selling, and using souls ("The Soul Trade").