From his lore:
"It is something they pray will not come to pass, for they know of no way to stop him." And:
"Few now remain who remember his earlier conquests, or know the true extent of his powers—but there are some ancient souls that do, and they fear the day when he may return to claim dominion over both the living and the dead."
So basically if Mordekaiser ever finds a way to come back, he'll be the the strongest being in lore with no one being able to stop him.