Tested some teeth:
Nice. That’s looking creepy. Glued them down and then spackled the edges so there’s a smooth gradient from the hardboard into the hill.
Once that was dry I sanded the crap out of the spackle to smooth it out, and started adding dirt for texture. I also sanded the edges of the face to make it look worn and older:
And more dirt / rubble!
It’s gluing in that last picture, which is why it’s very wet / gluey looking.