How is Dark Messiah connected to Heroes of Might and Magic?

I mean that Railag cursed himself voluntarily... Well, not in the sense of a curse, but in the sense that he simply changed... He even told Tieru about it: "You need to free yourself from the vile magic of Kha-Beleth." No one knows what's behind Agraila's mask. I know for sure that it's not a cute elf or a warlock, but a demon.