In singular they're called "a Stonechild," pronounced like you'd think but for some reason the plural form is "Stonechilds" as in "Rothschilds."
They're slow, so the only way they can catch prey is through numbers and ambush tactics. They typically lurk wherever large boulders and rocks congregate, lurking in a loose ring until some victims wander into the middle. Some scholars (the few who care to even take up the subject) suspect they were brought over at some point from the Elemental Plane of Earth, while others argue they are a native form of mollusc.
Disguised, a Stonechild looks like an ordinary stone, about the size of a large man's torso. This is actually their shell, though it looks and feels like rough granite shaped by aeons of wind and rain, often even sporting colonies of moss. The shape may be oblong and more forward-projecting or more balanced. From out of this shell when the Stonechild emerges protrude two stout, tortoise-like legs, which waddle furiously in splayed posture. Despite the effort both their speed and balance is poor.
From near the front or sides of their shell protrude two grey pipelike arms ending in blunt, four-fingered hands, the forearm and wrist swollen-looking, the skin rough as sandpaper. The arms are long enough that their fingertips can just meet if they reach around the front of the shell, but the Stonechild's body configuration makes wielding a weapon in two hands impossible. They carry stone maces, which some speculate the Stonechilds form naturally inside their shell analogous to how clams form pearls. The head of the mace can take many forms: sometimes a simple orb, othertimes a little pagoda of layered flanges. It is quite heavy and quite effective at breaking bones. They wield it one-handed with the other arm held out for balance or to catch them if their flat feet stutter.
Stonechilds are only about as high as a man's waist. Thus they usually swing for the legs, and full-on a blow from one of their clubs can disable a man's leg even through armor. They bring their targets down to their level with repeated savage blows. Then the Stonechild opens its mouth: a heretofore unseen crack in the stone pops up with a hiss of escaping steam and the squelch of mucus strings and a vomitous smell. Enormous teeth line the void with a thick tongue, the entire front section of the Stonechild's shell apparently devoted to its mouth. When their victim is well pulverized, preferably beaten into a kneeling position with head conveniently lowered, the lead Stonechild opens its maw and bites off the victim's head. Simple and effective. Brains really do seem to be the most prized part of any prey. They will work whatever chunk they tore off in their giant mouths with slow grinding and cracking, spitting out lumps of twisted metal armor and fragments of bone for days. They can go a long time between meals. They are just clever enough to leave any valuables from previous victims strewn around the scene of their next ambush.
They can speak. Terran, which lends some credence to the notion that they are outsiders, and in some cases a few crude phrases in the common tongue. Sometimes as they encircle prey, in their droning buzzing voices they pretend to be robbers. They say they'll the victim pass if weapons and goods are surrendered. At least until their ring gets close enough to charge (10' for their stumpy little legs). Sometimes Stonechilds just open those huge, flat-toothed mouths of theirs and scream for no apparent reason.
It's speculated they reproduce through eggs, which grow a suitably-sized rock shell over and around them as the fleshy creature inside actually finishes gestating and growing over centuries. As the abominations are thoroughly exterminated anytime a colony is found anywhere near civilization, much of their ecology is left to speculation. They are dumb and their senses limited enough you could play Jane Goodall with a circle of them if you really wanted, but it would be one boring adventure.
Products of breeding experiments, they are a subrace of Man. Perhaps once human slaves of the Illithids, crossbred with a tribe of blind Morlocks. Perhaps their gracile, sculpted bodies were the product of some feverish Drow fleshcrafter-aesthete. Maybe it was some asshole wizard who's name is now deservedly lost to time. In any case, they got loose and bred true and now they are a viable if gross underground culture.
Tall and wane with knotted muscles, they rarely stand to their full height but hunt and root through their caves and tunnels stooped, necks projecting vulturine from tensed shoulders. Their skin is an almost purplish grey, unkempt bristly hair soot-black. There may be some nods to decoration: headbands made from cave viper hide, hair stood up with animal fat. They have recessed noses, the nostrils almost slits, and heavy brows beneath which the eyes are intense black orbs, almost all pupil; the eyes barely work anymore. If you shone a torch in one's face he would see you as no more than a hazy ghost haloed in alien luminescence. Their mouths tend to project out from the face; any alien object will get a quick tongue-bathing to determine its nature.
They stalk the corridors and cliffs under the earth by probing the stones with fingers and feet. Their grey skin, corded veins standing up beneath as from intense dehydration, is tremulous. Their shoulders quiver, their too-long and somehow too independent fingers dance over everything and feel everything in an unsightly way. They have replaced vision with vibration, their tactile senses increased a hundredfold. They'll feel your footsteps in the stone even though you think you're being totally silent. You can't disguise your weight, the gentle easing and pressing of mass as clear to them as if you were touching skin. Their feet are long, balanced to keep the heel off the ground, with toes nearly as articulated and long as fingers; it's through the pads of these delicate extremities they can feel every shift in the movement of their prey. Their feet are their eyes and they take precious care of them. Pain overwhelms their senses: they avoid melee combat at all costs. Stone axes weighted for throwing, the hafts bone or stems of the ironshroom, and sinew-drawn bows are their weapons. Some wear carefully hardened mud-armor, always decorated with a spiral symbol in drawn ash. The spiral represents the winding endless underground; it is both holy symbol and map of their world. They use it on their boundary-stones. A drawn symbol may seem bizarre for a sightless culture but they remember they themselves were once sighted Men, and they know you can see it. At the center of every Under-Man community is a pool or sump with fresh water. This is their lifesource, their shrine and their retreat. They rarely sleep (once every 48 hours, roughly) but when they do they need the muffling of the numbing cold water on their hides; they sleep floating on their back in the still water and enjoy a respite from the constant bombardment of their heightened touch.
Most Under-Men are cannibals (in the sense of eating other human subspecies and sapient underground denizens). The few who aren't tend to live nearer to the surface, where they can trade with explorers or benign communities such as the Svirfneblin. These nearer tribes can make excellent guides for explorers from the surface, although your booted footfalls will be so annoyingly loud---the crunching, the thumping---that only an Under-Man gifted with patience can do the job.
Most, however, consider noisesome intruders only good for eating. They will stalk you by your vibrations, well outside the range of lantern or torch. At an opportune moment--when your party has just triumphed over some great beast perhaps--their flint-headed arrows will sail into your light, black missiles announcing the Under-Mens' arrival. They approach shooting steadily, a bend of cavemen boxing in dangerous animals, and when close enough throw their stoneheaded axes to break your skulls. At last remove a few will have longspears to try and keep the enemy's "beaters" from closing in on the killers. If their crude weapons can't break an enemy's armor they will withdraw, clambering up rough walls to escape and return to their slow hunt, awaiting the next chance to strike. They can do this again, and again. They're endurance hunters on their own ground, and they can see every move you make echoed in glancing impacts off the vaulted walls.
I got the name "Stonechild" off a monster name generator. There's really something to those monster name generators, you know? I'm not so good at coming up with things ex nihilo but having only a name provides a kind of prompt that gets the creative juices flowing. Under-Men weren't inspired by anything directly I THINK although False Machine should definitely be mentioned. It's similar to something he wrote years ago that I can't find but I think I made the idea my own (more quotidian, less poetic, I think that's basically my stock in trade) I can put this up with a minimum of gall.