User blog comment:Ado edem/Next ultimate skin?/@comment-5889036-20130609112106

My guess is Mordekaiser.. follows that dark theme as well as all the clues given by Morello.

Also,  Mordekaiser is very popular in LoL artwork and outside the game.. such as on RP cards and in cinematics etc.. he's very much a big icon for this game. He's a very likely candidate I'd say.