The ghostly cloak is a piece of members only armour. Players can obtain one after speaking to Viggora during the Curse of the Empty Lord miniquest. The cloak, according to Viggora, is supposedly drenched "in the blood of a thousand foes."
There are no requirements to wear (though the requirements and completion of Desert Treasure must be met first) and, when worn with the other pieces, the player appears transparent. Multiple pieces may be obtained through the drop trick.
If lost after obtaining it, it can be purchased from Perdu for 15,000 coins.