Originally Posted by
Lolretadin
Ner'zhul ceased to exist when he and Arthas merged to form the Lich King. He was intending to be the dominant personality, or soul, but Arthas rejected him and forced his conciousness through. When Arthas was the Lich King there was Arthas, the Lich King, and Frostmourne. The Lich King was a fusion of Ner'zhul and Arthas' souls and is an entity in itself, and has become the most powerful psychic entity in Warcraft lore. Frostmourne could be argued as to having a consciousness itself, since most of what Arthas said pertained to Frostmourne and what it wanted (ex: Frostmourne hungers). And Arthas' consciousness was trying to restrain them.
The ending to the Lich King was annoying, however, because the lore added in to the Lich King fight shows that he was the single most powerful entity on Azeroth, even moreso than the Old Gods. How you may ask? First is the fact that his army was more powerful than any other army on Azeroth. If let loose, the Alliance and Horde would quickly fall. You may, also, mention the Old Gods. But, unless you failed to notice, the Old Gods armies are only mighty on Azeroth because of the mortal races doing their bidding. If the mortal races were destroyed and the Old Gods emerged into a world that contained nothing but the Lich King's minions, lets just say they would not be alive in their sense of the word much longer. There is also the fact that Arthas, if he had wanted to, could have killed Tirion Fordring and all the other heroes in Icecrown Citadel with not much more than a quick spell cast that takes no longer than it takes him to swing his sword. No other entity in Warcraft could do this lorewise, not even the mighty Kil'jaeden. So, arguably, he is the most powerful 1v1 combatant as well because his power over death allow him to kill anyone he sees fit to.