The Ghost is a non-player character that appears during The Restless Ghost quest. It can be spoken to at any point before or during the quest, however the player will need to equip the Amulet of GhostSpeak to do so. It is obtained from Urhney, who can be found in a shack within Lumbridge swamp.
During the quest, the player uses the amulet to converse with the ghost and is asked to retrieve the ghost's skull, which is found at the altar in the Wizards' Tower basement. It will stop talking to you after the quest has been completed.