what exactly will happen to Arthas, but it's looking pretty likely that we are going to be able to actually kill him in Wrath of the Lich King. This raises another interesting question however: Sincast asked over on the forums, "When Arthas is killed in-game does that mean that undead will be no more?..." Nethaera popped in right away to answer him... sorta:
Interesting concept to think about isn't it? But does killing Arthas really mean the undead cease to exist? They existed when he was just a paladin didn't they?Although we have seen that Arthas and the Lich King are now "one," it remains to be seen what "one" really means for them. Are they two souls in one body? Is Arthas the flesh and the Lich King his armor? What kind of state would the Lich King exist in after Arthas was killed? What sort of threat would the Scourge pose at that point -- just fodder for new characters leveling up in future expansions perhaps?
There are things to ponder here but most importantly...
Does the Lich King cease to be when the vessel is destroyed?