The ghost has already slaughtered some adventures who tried to defeat it, so no one dares to go near it.
If the Rune Warrior engages it the ghost will use melee attacks and lifetap to regenerate health once damaged.
However the warrior itself can be easily overwhelmed.
Once the ghost is defeated he will have two items at his corpse: An old rusty sword and a strange ring. The sword must be shown to Brok to prove the ghost is defeated and for completing the quest.
When showing the ring to Brok, this will start the quest From the Deep.