To start Restless Ghost speak to Father Aereck in the Lumbridge Church. Tell him you are looking for a quest and agree to help, he will tell you about a ghost in the graveyard. He will refer you to Father Urhney, who lives in a shack in the southwest edge of the Lumbridge swamps. Find Urnhey and tell him that Aereck sent you. Tell him about the ghost in the graveyard and he will give an amulet which allows you to speak to ghosts. He will tell you that most guests have a reason for staying and you may be able to solve it's problem.
Ghostspeak-
Head to the graveyard below Lumbridge church and go inside the building. Equip your amulet of ghostspeak then open the coffin, the ghost will appear nearby. Speak to him and he will be amazed you can understand it. Then after a bit of questioning he will mention that his skull is missing. He will also tell you that he was attacked by a Warlock while mining south of Lumbridge.
Find the skull-
Head to the mining area south of the graveyard and search the rocks to the East of the mining spots to find a Muddy Skull. Then a level 11 Skeletal Warlock will begin attacking you with magic. Either kill it or run away and then head back to the graveyard. Open the coffin if it is closed and use the muddy skull on it. A cut scene of the ghost being freed will play and then the quest will be complete!
Rewards: 1 Quest Point 1125 Prayer XP Amulet of Ghostspeak
