Exar Kun was an idiot Jedi that went dark-ish...his story is a little ridiculous actually. Basically he got lured in to a cave and a ghost wouldn't let him leave until he turned evil then shenanigans and eventually he's betrayed by his pupil and destroyed...as a ghost which shouldn't even be able to happen. That's the very short version. You should google him. It's worth reading his wiki page.