There is a world far away populated by graceful and beautiful winged beings gifted with immortality, where an ancient conflict still rages. Like so many conflicts, this war split families. One side proclaimed themselves as beings of perfect order and justice, fighting to unite the world under their law and strong central governance. Those that fought against them saw their kin as tyrants, creatures incapable of seeing the larger view, who would sacrifice individuality and freedom for the illusion of efficiency and safety. Morgana was one who fought against what she perceived as the tyranny of her kind, and for that she was branded fallen. Morgana was not innocent, having plumbed forgotten ways to gather forbidden might to become a powerful mistress of the black arts. This goal was driven by her obsession to defeat the general of the opposition's army - her sister, Kayle.
While the two were in fact birth-sisters, Kayle struck the first blow by disowning any filial connection when Morgana refused to join her cause. Eventually, Morgana grew in power enough to not only reach, but challenge Kayle. As the time approached when the two would meet in what could be their final conflict, Morgana was suddenly summoned to Valoran. At first, Morgana made a deal with the League's summoners to fight in exchange for greater power. With the advent of Kayle into the League, Morgana now willingly fights in the League of Legends for the privilege of being able to destroy her sister again, and again, and again. She lies in wait for the day the bonds of the Institute of War no longer hold her, and on that day she plans to destroy Kayle once and for all and return home a hero.
Source: The Publisher Website.