The Little key is an item used during the Plague City quest to open Elena's cell. It is obtained by searching the barrel in the upstairs room of the house in West Ardougne where Elena is currently being held. You cannot enter the house until you have obtained a Warrant that is given to the player by Bravek, after which it is not needed to access the house. After opening the gate, the key is no longer needed and can be opened without the key.