Backstory from the Game Guide
According to the backstory from the game guide, it's Leith Torles who made Iskandar into "the Unlimited" with his secret arts. Iskandar then left this world and began wandering from one world to another. "The Unlimited" can come and go between the worlds and is also invincible and immortal in all worlds except where Iskandar was born. He grows old only in the world of his birth.
This is rather interesting, as it makes Leith Torles to seem like an even more impressive sorcerer, although it's still not entirely clear if Leith was aware if his spell was successful. However, judging from the cutscene in Ruby's vision, it is clear that Leith had a vision of the future, so that would even add fortunetelling to his set of skills.