The stone is quite hard, and it may take a few whacks (edit, yes, the attack above just missed...I'll just assume he tries again). Forge also gets some eldritch backlash (damage as rolled).
However, with those whacks, the outer stone cracks, and beneath it is the carved eye with pupil as on the devil face back in the first tomb!
Then you realize (with all the high perception scores standing about) the dark sphere in the mouth seems to vanish!
But.....almost as soon as the eye is revealed, the outer stone starts to grow back!