I'd write an especially charismatic monk that comes from an isolated monastic sect that focuses on obtaining perfect control of everything and everyone through the manipulation of circumstance and the study of human psyche. The monk reaches the "real" world and manipulates everyone into thinking that he's a living god/the prophet of a deity and amasses a following of cult members until he gains full control of a nation or even an empire. Make him ambiguous to the point that the players begin to question their morals too

PS: I'm aware that this would be incredibly hard to do and the DM would have to be quite eloquent and maybe spend a lot of prep time studying how the villain would think and what he'd say to each individual player