May be that Kayn was inspired by Wakfu
There are these two MMO's called Dofus and Wakfu, which are somewhat of brother games, sharing the same universe and lore in different timelines. The IP also has spawned numerous other media such as other games, and two TV series, a movie and an OVA, which are probably the most popular part of the IP.
So, in this universe, the demons are creatures called Fab'huritu or Shushu, which can sometimes cross into our world and are encased into items such as (but not limited to) weapons. These are asigned to a guardian who must keep them encased all the time and carry the item with them always. But these guardians can sucumb to the Fab'huritu and be posessed by it, becoming some sort of amalgamation between the two, in control of the demon.
When I first saw Kyan and Raash, this was what came to my mind. I'm just curious, I would like a rioter to confirm my thoughs, because as a fan of the Dofus/Wakfu IP I would be so honored to know that this concept inspired a new champion which I'm totally gonna main.