You know, Imp Natural Attack doesn't even list 1d12 as a damage size when determining how to improve the weapon? And manufactured weapons treat d12s as 2d6 for that purpose.
Based on the slam damage, I'd say make them fairly strong, huh?
I think they just trap people with stone shape and earthquake. Wait, they don't have stone shape? Seems a natural--better than earthquake, actually, which is quite deadly for the CR of these guys.
Yeah, earthquake seems pretty nasty. I agree that stone shape makes sense, and I'm willing to consider dropping earthquake. Shade, what are your thoughts?