Just happen to know this for some reason. xD
Assuming it's the copper key I'm remembering..
You find that key somewhere down in the ice islands like floor -3?
I think it's guarded by a minotaur guard and some other mino's..
It's used over in senja at the castle to open the door with the 3 mino's inside.
Alawar is a crazy fellow for sure.
Losing his keys everywhere and guarding his treasure..
Such a sad story.