The Legendarium Key was sealed forever with the legendarium. So the legendarium key can't unlock due to the fact that the key only locks the legendarium FOREVER. Hope this helps you!
The Legendarium key may possibly be able to unlock the Legendarium. But you can't blast it open that's for sure. Maybe you could also destroy it. But maybe, just maybe, the key can unlock the Legendarium as locked it before at the end of season 6. So, maybe.
