This does not work for me? Is it an RNG drop despite the quest step? I have done several runs in a row and have not gotten it. It says to just complete the forge, so I don’t know what to do...
You gotta open the box once you beat it
You need to complete the quest Ada gave you where you had to pick up observator lenses on the Leviathan. That's the only way to have that key because you need to recreate it before having it on Bergusia. Next time, google the Izanagi's Burden quest