The new League of Legends Champion actually consists of two entities. Kayn, the Initial form, wields an ancient weapon and fights the dark within it, Rhaast, for control. During each game, one will consume the other. Kayn hones his shadow skills by hunting ranged opponents, while Rhaast fuels his bloodlust by going toe-to-toe with melee enemies. If Kayn wins the struggle, he ascends to become the shadow assassin.
So basically Kayn is the initial champion and Rhaast a demonic entity.