While soloing in Wajaom Woodlands, I got this drop from a Woodtroll Ranger. It dropped just once in the total 150 minutes or so I've spent in the area, so prolly not the best drop rate on those mobs.
If you're on Hades, which loses half of its Besieged battles, and your Warp Tarutaru gets taken prisoner in Halvung you can use this key to spring the little fellow from jail.
I'm not sure what drops it (probably the truesight Trolls), but it opens the hanging cages found in Halvung.