Ghoulish mask was removed from RuneScape after an update, but still exists in-game for those who had obtained it.
A ghoulish mask is a rare item that could be won on the Squeal of Fortune and Treasure Hunter. It has an emote that transforms the player into a ghost, during which time the player may not move or perform any actions other than speak. It also has a right click option in the equip screen that allows you to take a forum picture as a ghost.
It was once again made obtainable during the Blast from the Past II Treasure Hunter promotion on 17 July through 22 July 2014.
When trying to perform the transformation emote of the ghoulish mask next to an Evil Tree, it will not perform and you get a message that states, "The magic from the nearby evil tree could make the effects of the transformation unpredictable."