From wowwiki (sourced from the books):Originally Posted by Aldarc
Nozdormu has both a power and a curse: he knows when and how he will die. Until that time comes, he cannot perish.
Meaning he can't die until his "determined" time of death. We don't know when this is. It could be tomorrow - most likely not, however. Either way, up until the time of his known death he is completely and utterly indestructible.
So if Neltharion has the Demon Soul, he wins (because it contains part of Nozdormu and can control him). If the Dragon Soul is out, Nozdormu wins because he simply is unable to die.