The brass key is the key that is obtained in the Edgeville Dungeon and allows the player to enter it just north of the Hill Giant room via the shed with the locked door. The player only needs to be holding the key in their inventory to get into the shed located west of the Varrock west bank.
To get the key, you must enter the Edgeville Dungeon through the back way in the entrance south of the Edgeville bank and make your way through the dungeon. Just north of the hill giant room, there is a small circular room with the key on the ground. It re-spawns at about every 45 seconds.
This key can be placed on the steel key ring.