This key unlocks the door in the Taverley Dungeon, just past the Lesser Demons. To obtain the key, go to the southern area of the dungeon, with the Black Knights and Lord Daquarius. Kill the Jailer; use the Jail Keys he drops to unlock the door with Velrak the Explorer in it. As a reward for freeing him he will give you this key. There is also a Dusty Key respawn near where the Lava Eel fishing spot is located and is bought at the Legends' Guild for 1gp.
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If you are talking about the bit the leads into the Dragons area, you will need a dusty Key, one of these can be obtained by going to the Room Full of Black Knights then On the East side of this room is a Jail Cell. To enter you must kill the guard outside, take the key he drops and use it with the door. Once inside you can talk to the Explorer who is locked inside and get the Dusty Key.
when you go in go past the moss giants and into the black knights hideout to the left of it there's a jail, kill the guard and he should drop a key, (THIS IS NOT THE KEY YOU NEED). Use the key to get in the right hand jail cell and talk to the guy for a key named Dusty Key.
you can find sinebeam trees in Daemonheim on runescape
Buy it, get it from the Hill giants.
open the dragon chest in Taverley