Kayn was born a Noxian — one of the prominent nations of Runeterra — and was conscripted during the war between Noxus and Ionia, which is a nod toward the in-game event that pit the two factions against one another in the past. Abandoned and weak, Kayn was discovered by Zed, who added Kayn into the Order of Shadows.
As an acolyte of the Order, Kayn grew up as an assassin and learned under the tutelage of Zed, until news of Noxus uncovering a dangerous weapon from Ionia reached the Order. Perhaps in an attempt to keep such a strong weapon from falling in the wrong hands, Zed dispatched Kayn to deal with the weapon before it would reach its destination. On his way, another acolyte joined Kayn before the caravan neared the ambush point.
Kayn ambushed the caravan holding the weapon and killed the guards, but one unleashed the weapon and started to madly attack his own Noxian allies clearly possessed by it until Kayn finished him off. Instead of destroying the weapon, Kayn elected to wield it for himself against the pleas of his fellow brother of the Order. Kayn then used the weapon to slay his brother in arms and uses the weapon still, locked in an eternal battle for control against the weapon’s own consciousness named Rhaast.
It’s been confirmed that the Rhaast is part of the Darkin race, which is also represented by Aatrox, another Champion in League. Not much is divulged into what Rhaast actually is, other than the fact that it wishes to be free through access to a host body, and Kayn doesn’t relinquishing control easily.
The struggle between the two is reflected through his unique in-game transformation gimmick, which allows Kayn to either evolve into a Shadow Assassin by full controlling Rhaast, or by turning into a Darkin, giving his body up. The transformation is triggered based on how many ranged and melee champions Kayn kills, with ranged champions giving him charges to become a Shadow Assassin, while melee champions allow him to be possessed by Rhaast.
Cover photo courtesy of Riot Games