The player can get the key from Velrak the explorer, who is imprisoned in the Black Knight headquarters in the southern part of the Taverley Dungeon. Kill the jailer to get a key to enter Velrak's cell and talk to him, but do not ask for a reward!
One dusty key spawns deep in Taverley Dungeon, near the Lava eel fishing spot south of the blue dragon area. Note that this key spawns in the deeper area of the dungeon, so players who need a key to access the deeper dungeon cannot obtain it here. Players with level 70 Agility, however, can obtain this key by using the agility shortcut near the entrance to the dungeon. The shortcut goes to the blue dragon area.
The dusty key is not tradeable, but multiple copies can be obtained. It can be placed on the Steel key ring.