Elementalist Lux, and Kayn.

What do these 2 have in common?

Well, the ability to change their appearance (and for Kayn, his kit) in the middle of a game. However, you can only change once you have damaged your enemies enough, and once you make that change, its permanent for the rest of the game.

Kayn may very well have not been released in this fashion were it not for Elementalist Lux. Riot has said in the past that Ultimate skins are a way for Riot to go balls to the wall, and challenge themselves in every way possible, while also introducing new tech that they can use further down the line. Pulsefire Ez lead to changing models in game (which helped pave the way for future Ultimate skins, and some legendaries as well). Spirit Guard Udyr challenged Riot particle and model wise (you know how Garen's ult makes the cool shockwave after it lands? Yeah, thank SGU for that), and the ability to change models at will (which helped for both DJ Sona and Elementalist Lux). DJ Sona not only introduced new music to the rift (which may have helped pave the way for the music you hear with Jhin), but also a unique color changing mechanic (Arcade Ahri's tail was made because of DJ Sona's hair. If you look at Arcade Hecarim's tail, you'll understand why they needed this mechanic).

And finally, we come to Elementalist Lux, the first skin released to have the ability to change based on your interactions in game. Were it not for this mechanic, Kayn may very well be a completely different character, or, just a normal transform champ, like Jayce or Elise. I always did wonder what kind of tech they could use from E. Lux in the future, and today, I got my answer.

If you were ever wondering why Riot goes through with making such badass skins as the ultimates (of which, they actually make very little, if any money on), you need not look further than Kayn. The Ultimate skins are there to push Riot to their limits, both tech, and artistic wise, and I'm eager to see how they push themselves on the next skin.