Kayn

A peerless practitioner of lethal shadow magic, Shieda Kayn battles to achieve his true destiny—to one day lead the Order of Shadow into a new era of Ionian supremacy. He wields the sentient darkin weapon Rhaast, undeterred by its creeping corruption of his body and mind. There are only two possible outcomes: either Kayn bends the weapon to his will... or the malevolent blade consumes him completely, paving the way for the destruction of all Runeterra.

Source: https://universe.leagueoflegends.com/en_US/champion/kayn/