Ostarion's Revenge

Ostarion's Revenge is a kill quest given by Argent. It requires killing a Boss mob. It is preceded by the quest King of Bones.

How To Achieve
Part of a "story", this quest follows the first quest against Ostarion, King of Bones. This quest is the same as its predecessor in the sense that you must kill Ostarion again, but this time, Ostarion will be much stronger than before, he drops no weapon and he will respawn after 2.5 seconds if killed at night.

So the strategy is to visit Ostarion during the day. It doesn't matter that he is in a cave, what matters is that he will respawn at night regardless of the light level in his lair.

Once he's killed again, return to Argent to claim your reward, and finish the quest.