Howling:Series 10 Vault

With the given information I want to take several guesses as to what's inside this obscure vault that the Doctor and Nardole are apparently protecting. Obviously this hasn't been revealed yet so this thread is purely speculation. My first guess is, John Simms character - The Master. From the trailers we can see that they will bring him back this season at some point, so I think a perfectly logical way to do that would be via him making some grand escape from this mysterious vault. What else could be so dangerous that Nardole insists on the Doctor staying "on world" so that he himself can watch over the vault?