This...big, fancy keys. Alright. This makes exactly no sense but Envy can see what the spontaneously appearing weapon is doing to the creatures.

He's not going to question it too much, there's not time for that.

But what he is going to do is do some serious concentrating of his own, just hoping that it's going to work in this odd dream-state...

And as he concentrates, his right arm takes on a glow of its own, a crackling white light starting at the elbow and traveling upwards. And when it's done...well, it's not a full one, but he sure did turn his forearm into a key that looks something like the one Vanitas just pulled out of nowhere. He has no idea if it will work, but he takes a swing at the monsters all the same.

"Ha! That's it, show them who not to mess with!"