I've made discussion about kayn, but i didn't think enough to make him balanced.
Class origin: Assassin(and maybe blademaster idk)
Cost: 1 gold
Armor/Magic resist: 20/20;
Attack Speed: 0.55
range: 180/1 hex
Ability cost: 100 mana
1st Ability: his ult in the other modes;
2nd Ability: By making 2-star kayn you can make him demon(darkin, not the item), or wait till you get 3-star kayn and make him phantom(shadow assassin).
Darkin's effect: Heals for the 40/60 percents of the damage he deals, but healing items' effects are 50 percents effective.
Shadow assassin's effect: when the fight starts 2 random enemy champions are marked. If marked champion dies, the mark goes to other champion(can't be stacked). Kayn deals 2 times more damage to marked enemies.