I feel like Riot missed a huge opportunity for an amazing character, with Kayn
Here's what I think SHOULD have been:
Kayn is an assassin, and a very good one at that. There was only one problem: He was extremely lonely. Anyone that he befriended was soon killed, as to not let himself get caught. He couldn't trust anyone. But then he found Rhaast. A darkin scythe who would grant immeasurable power to the person who overcomes Rhaast's demonic power.
Kayn didn't even hesitate when he found him. He swiftly approached Rhaast and grabbed his hilt. The battle for Kayn's body began... Or so Rhaast thought. Kayn wasn't seeking power. He wasn't seeking vengeance. Or anything similar. These factors are what allow Rhaast to control his host. But, every time Kayn killed someone, he didn't feel like that at all. To Kayn, this was all a bonding experience. For, Kayn had finally found a friend. An evil friend who didn't really like him, but a friend regardless.
"You ready best buddy?" "Sod off" "The best."
"Hey Rhaast, we're friends right?" "No, we're not." "Aww, I love you too buddy."
After Rhaast finally managing to take over Kayn's love filled body: "That kid was a little creepy."
"Hm... He probably would have loved to kill that person
After Kayn transformed into the shadow assassin: "You ready to knock some heads best buddy? ...Rhaast...? You there? ...Oh no..."