A key is an item found in the Waterfall Dungeon. It can be found by searching a crate in the eastern chamber of the dungeon. The key is used to open a door in the western chamber of the dungeon (the door between 4 and 5 on the map), and also the next door north of that (the door between 5 and 6 on the map). The key also spawns in the chamber between these two doors, allowing the player to pass back through the door if they were to drop the key.
It is not recommended to keep the key in the bank, since it is found near the door it unlocks, and access to the chamber is not needed after the Song of the Elves quest. It cannot be stored on a steel key ring.