(Updated lore, added extra details and revised sentences.)
They say that the Void can mess with your head, especially when you're exposed to it. Unfortunately, that's what happened to Kassadin, the Void Walker. It was said that Kassadin held an immense power in him that would be truely shown if the effects of The Void would take him over. Hearing many tales and folklore about the place, Kassadin refused to go even when his head was mysteriously telling him to. It was then until his beloved daughter, Blair got kidnapped by an unknown being and was no-where to be found ever since. It struck to Kassadin who took Blair, having him immediately know who took her and where they took her to. Malzahar, the Prophet of the Void, took her, in attempts to lure Kassadin into The Void to get his precious daughter back. Forced to be sent to the Void, Kassadin set out and finally gave into going for the sake of his daughters life. But what he saw changed his being forever, he saw terrible creatures in The Void, just very terrible things that would rip the innocence out from under anyones skin who were once pure, turning them into gruesome and terrifying beasts -- physically, and mentally. From that day, something inside of him changed, and it eventually took all of him over, changing his appearence and personality completely. After he entered the Void, Malzahar vanished and was no where to be seen. Kassadin left the Void in search for Malzahar, forgetting all about his daughter being trapped there. Waiting for the rescue of her father in The Void, Blair, who was a scared and lost child cried and screamed out every night for her father. No one ever came to help get her out of this terrible place. As she grew up, her hatred for her father grew aswell. There was a creature in The Void she met the first day she was sent there, it was an ancient and wise being. It was called 'The Eye' and in exchange for her soul, it told her it would get her out of The Void and help her find her father to get the needed revenge from his betrayal. Being such a young girl at the time, Blair of course had agreed and wrecklessly signed her life away to her protector, her savior. Ever since then, both 'The Eye' and Blair are connected, becoming one. But unfortunately, the exposure from the Void messed with Blairs ability to walk, paralyzing her from the waist down. So for transportation and support, Blair sits ontop of the creature, also being able to control it's movement since they're now connected. 'The Eye' is her only friend, the only thing she knows will protect her from any other evil that would try to harm her. When Blair turned fourteen, she finally set off, having 'The Eye' gain enough power to open a gateway out of The Void into Runeterra. As soon as she escaped she knew exactly what she was going to do first; she was going to kill Kassadin for betraying his own daughter and to kill her kidnapper most importantly. 'The Eye' would help her by doing this, it will help her with anything in exchange for her soul.