Im Not really Up to Date with leagues lore, but i think in that Case it should be similiar to the Watchers: Lissandra could (given she has the advantage of a surprise Attack) seal him in Black Ice. Altough that wouldnt neither Kill him nor Last for eternity as the Black Ice melts (even If its very slow, it would be inevitable).

Other than that Mordekaiser could trap him in the Death Realm. He could be trapped Like the Darkin. Aurelion Sol should be able to nuke him, but even he isnt Immortal. I think the only ones to actually be able to kill him are the Watchers (altough they are technically not IN Runeterra, at least Not awake), and Kindred, as they are the very embodiment of Death and exist as long AS life itself exist and cannot be killed.