Kayn Ability (LoL): The Darkin Scythe

Kayn wields an ancient, sentient weapon that battles with him for control. Dealing damage against champions causes them to drops orbs that are automatically collected upon leaving combat, filling a bar above Kayn's portrait.

Once the bar is full, he can choose to either be overwhelmed by Rhaast or expel him to become the Shadow Assassin, based on whether he has damaged more melee or ranged champions, respectively. Once primed, the other option becomes available after 4 minutes. Kayn can continue collecting orbs for his other form to reduce the wait time.

The transformation can only be performed by interacting with Kayn's portrait while on the summoning platform, permanently changing his abilities.