There is a place between dimensions and between worlds. To some it is known as the Outside, to others it is the Unknown. To most, however, it is called the Void. Despite its name, the Void is not an empty place, but rather the home of unspeakable things, horrors not meant for minds of men. Though such knowledge is lost in modern times, there are those who have unwittingly discovered what lies beyond, and they have been unable to turn away. Kassadin is such a creature. He was once a man forced to look upon the face of the Void and forever changed by what he saw. Once a seeker of forbidden knowledge, he discovered that what he sought was something else entirely. He is one of the few that has found his way to forgotten Icathia and lived to tell the tale, following the scant breadcrumbs hidden in ancient texts.
Within a decaying cyclopean city, Kassadin found secrets of the kind that he will never share - secrets that made him quake with fear at the visions of things to come that were thrust upon him. The power of the place threatened to consume him forever, but Kassadin took the only route available to him in order to survive - he let the Void inside him. Miraculously, he was able to overcome the alien urges that went with it, and he emerged as something more than human. Though a part of him died that day, he knows that he must protect Valoran from the things scratching at the door, waiting to get in and visit their torments upon the world. They are only one step away... something to which the appearance of the abomination known as Cho'Gath attests.
If you look upon the Void, you can't put it behind you. If you look upon Kassadin, he is probably already there.