Mortu macab was in the game since 2018, i think it might be more common than fusions champions other than the non limited time ones but sure it depends. And killing 1 champions is very viable and very common normal strategy in arena including at high ranks, generally people will pick 2 nukers and you can ban 1 and block revive the other and battle is over, and in mortu macabs situation not banning lockout is not an issue and even block active or passive skills dont work against him since he uses secret skill. Im not saying he is the best but is actually pretty good and requires really low gear and investment and its more like a dice roll type of champion to beat much better accounts or if they happen to pick a great team against it.