We've never actually had an origin story for the legends of Ravana. Encyclopedia Eorzea theorizes that he's the gnath's ideal of the perfect warrior-god, but notes that the Onemind's inherent hostility towards outsiders makes it impossible to actually ask. He does bear a resemblance to (and share a model/skeleton with) the Phantom Ray boss from the Fractal Continuum dungeon, but no actual connection has ever been made.