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You get it when you kill a dragon.
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.
Taverly Dungeon, as part of the Heroes Quest, just past the Blue Dragons and smack bang in the middle of the Baby Blue Dragons. ( You will need to fish in lava - use an Oily Fishing Rod )
the karajama dungean has bronze along with red and blue
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.