Muddy skull is a quest item used in The Restless Ghost. It is found in the Lumbridge Swamp training mining site, south of the graveyard in Lumbridge by searching the large rock near the ore. After searching the large rock, a level 11 skeleton warlock will appear. Killing it is not necessary, and you may run if you want to. Putting the skull back into the restless ghost's coffin will complete the quest. It used to be inside the Wizards' Tower basement (as a ghost's skull) on the altar, but due to an update they sent it to the Lumbridge Swamp mine.
The muddy skull looks exactly the same as a skull which can be obtained in the Wilderness.
If you use the Muddy skull on the Restless ghost will say "I can't give it to him. It goes right through him."
In , it was possible to summon skeletons by dropping the skull and picking it up again. Some players used this method to kill low level macro users.