Path of the Demon (D&D barbarian subclass) (5e)
Barbarians that follow the path of the demon in one way or another have their soul bound to a demon. Some are the products of abyssal cults¸ some have been possessed in some way that still allows them agency¸ no matter how they got their demon¸ their rage is fueled by the demonic strength and abyssal anger that persists in the reassesses of their soul.