You must kill Archmage Galus and return with his Staff from the Cursed Depths, just east of Baradin Hold.
We have been trying to clear out the Cursed Depths cellblock since we landed on Tol Barad. It seems that everytime we get close, that archmage shows up and pushes us back.
Now that we have claimed the island, it's time we take another shot. Head down there and bring me back his staff as proof that you have finished the job.