Shaelmaar

"The war council, weary from clearing the rubble, advanced through a tight series of dirt tunnels beyond Silverpine Watch. A musty, damp smell filled the air, and the tunnel opened into a pitch-black cavern. A high pitched shriek echoed through the caves, followed by a rumbling. Stalwart Tom barely avoided what appeared to be a rolling boulder, while Noro was half-buried before a rock slide. The blows of Caspar and Gerhardt glanced harmlessly off the creature's armor, while Ivan's flame only seemed to anger it. A desperate struggle began as the monsters rolling charges collapsed sections of the cavern and crushed the party in its wake. At last, the beast was flipped by the half-orc monk Garon, and the blades of the War Council put the Shaelmaar down."
A shaelmaar is a rare, subterranean creature that dwells in the deep places. It vaguely resembles a massive mole covered in rock, around 10 feet tall and wide. It can detect enemies through tremor-sense, and ferociously defends its territory by rolling towards enemies and crushing them under its massive bulk. 
In the long lost outpost of Silverpine Watch, the war council discovered a shaelmaar beyond a section of collapsed tunnels. They were able to defeat it only after knocking it on its back and exposing its soft belly.