You will find Fineous Darkvire beyond the Ring of Law, in the Hall of Crafting. Kill the miserable cur and recover Ironfel.
Take Ironfel to the Shrine of Thaurissan and place the hammer in its rightful place: In the hands of the statue erected in my honor.
When this is done, the compartment in which I stored the master key will open. More importantly, Ironfel will remain forever in my grasp. Should they make an attempt to remove the hammer; both the statue and hammer will shatter, lost forever.