But he's still quite on fire, isn't he?
The Lich King's ending was a smart move from the perspective of story telling.
At any time Blizzard feels like, the story can be picked up again, and spun further.
Even a babyface turn of the LK is possible. An unexpected showing up of the LK's armies to aid the known protagonists Horde and Alliance against an enemy so powerful that they apparently cannot overcome the threat they are facing.
Essentially it's shrouded for now, how Bolvar will cope with the task he took up... Will be preserve the good in him, because the light shields him and is strong enough. Or will he too fall to the darkness and evil, which would logically call a rise of the scourge once again.
We continue....... Stay tuned....
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Arthas (the LK) failed to break Bolvar. He (for whatever cocky reason) tries to turn Bolvar while he was still alive. At some point it appeared Bolvar was/is exceptionally gifted by the light, that protects him. He was not dead, when he took up the crown. He was beyond the point of recovery though. Eventually he would have died. That reflects in the dialog of the cut scene after Arthas' dead.