Skull Key of Erosion
Key used to remove the time restriction on instance and allows you to enter the Hall of Erosion. Can only be used once a day outside the entrance to the Hall of Erosion. Cannot use multiple skull keys daily. Cannot be exchanged or dropped. Can be stored in a private warehouse.
Not dropped by monsters.
Cannot be spoiled.
The chances are official (x1). On servers with x10, x100 and higher rates, drop and spoil come more often.