It is no longer possible to obtain a warlock cloak, or any other part of the warlock costume. During the 2008 Hallowe'en event, players had to go trick-or-treating in Rimmington, Port Sarim, and Draynor Village to receive candy from certain NPCs. For this part of the costume, four chocolate drops needed to be received. Maggie would give you the cloak when you had all the chocolate drops. In the members-only Costume Room of a player-owned house, the warlock costume can be stored in a toy box. If lost, the warlock costume can be re-obtained from Diango in Draynor Village.