What REALLY happens when Mordekaiser ult's Karthus?

Quote: "the demons of the Shadow Isles saw purpose in Karthus, and after his murder at the hands of Mordekaiser, imbued him with undeath before steeping his voice in the fury of countless lost souls."

Karthus is already one of Mordekaiser's "Children of the Grave", can he even do that?

I propose a change, when ulted, Karthus should come to life, as the dead cannot die, his defeat would only mean overcoming his death.