The restless ghost is the ghost you put to rest in The Restless Ghost quest. A ghostspeak amulet must be equipped to talk with him. During the quest, the player must retrieve his skull from the mine south of the graveyard. When his skull is returned, he'll say he can finally go to rest and will fade away. When the quest is completed, the ghost will no longer appear when the coffin is opened.
While he was mining in the mine south of the graveyard, he was attacked by a warlock and died. His head was chopped off, this led him to be a ghost and not able to rest in peace without his head in his skeleton body in the coffin. His head is in the rocks near the mine, after picking his skull, a skeleton warlock attacks the player, possibly the one that killed restless ghost.