oh hold up for a second.

I've been playing with the assumption the vampires can easily tell if he's bluffing. Whenever he hid anything or tried any form of deception I wrote about it so you can decide to act on it.

He isn't lying and Bedivere knows it. That stone is the real deal.
Martin doesn't expect the blast from the box to kill any of the vampires, but it will ruin the stone. And lastly he really is working for someone who send him along with all this stuf.

Also, Mithias and Llisia have seen Martjn not give a single fuck when other mortals were threatened. So there's a hint he isn't exactly on the human side.