The Restless Ghost Quest Guide
To start, speak to Father Aereck, who can be found in the church of Lumbridge, which is just east of the home teleport spot. Ask him if he has any quests for you, and it'll bring up the Restless Ghost quest starting form, as seen above. He'll then explain that you need to see a good friend in Lumbridge swamp.
Head south into the graveyard underneath the church, and there is a break in the fence that will allow you get into the swamp. Once in the swamp follow the path all the way untill you reach ores and the mining tutor. From there head west untill you reach a small hut. Go inside and you'll see Father Urhney, tell him Father Aereck sent you, and then say a ghost is haunting his graveyard. He'll give you a ghostpseak amulet, which allows you to speak to ghosts.
Head back to Lumbridge church graveyard and go inside the building there that houses a coffin. Open it up and the Restless Ghost will appear. Talk to him with your Ghost speak amulet worn. He'll say that he can't rest in peace because he lost his head while he was alive. The last time he remembers seeing it was when fighting a warlock to the mines south of the graveyard.
Make your way to the mine that you passed earlier, and search the rock that is west of the ores. You'll find a muddy skull and a Skeleton Warlock (level 11) will appear. You can either fight it or just run away. Go back to the coffin and open it up and use the skull on the coffin to place it back inside.
A shortcutscene will commence, and then you'll recieve your reward.