Arthas Menethil is dead. You see him die
The Lich King (Ner'zhul aspect) remains frozen upon the......well.......frozen throne. There he will be locked in a battle of wills with Bolvar Fordragon, until one of them eventually wins, spawning some lore, a novel, or a Warcraft game of some sort. Perhaps a WoW expansion, perhaps another Warcraft game.
Of course there's also the possibility that they might just stay battling wills forever, but this seems somewhat unlikely given the way that the Lich King has already managed to return once from being frozen in Icecrown Citadel. Past Blizzard form also suggests the return of the Lich King, with the hero of Diablo 1 battling against the soul shard he jammed into his forehead (like a tool). A battle which he lost fairly quickly, giving rise to the events of Diablo 2.
My opinion on the matter is that the next time he is seen the Lich king will either face his ultimate demise or redemption in some kind of ultra-apocalypse involving all manner of demons and Burning Legion types. Perhaps, if it ends with a bang, in the final WoW expansion.
PS - the possibility of redemption lies with a small point relating to the merged souls issue. Arthas Menethil was judged beyond redemption by Tirion Fordring. He said nothing of Ner'zhul.
Last edited by Klogaroth; 06-28-2011 at 01:34 AM.