Bunkerling

A bone-white, deathly-gaunt humanoid emerges from the darkness.  The creature has no eyes, but its mouth and ears are grotesquely oversized.  Clad in armor made from metal scrap and wielding an old, rusty M16 with an even more decrepit bayonet strapped to the muzzle, it looks eager to tear the flesh from its former cousins.

Bunkerlings are the descendants of the inhabitants of the Sunnyvale Civilian Emergency Shelter, an experimental bunker system constructed shortly before Zero Hour.  Harboring thousands of civilians and off-duty military personnel, the place was near capacity when a nearby missile silo was struck by a high-power ground-burst nuclear weapon.  The bunker functioned perfectly for about a year, until an unknown saboteur destroyed the bunker's door and air filter systems, trapping them inside while the vents pumped in huge amounts of radioactive dust.  Over the next few decades, each new generation born in this bunker would be less and less human, mutated further and further to survive and thrive in the underground.  The last recognizable humans gave birth to the first bunkerlings, who grew up rapidly to tear apart their progenitors and started digging huge caverns out with their bare hands.

Bunkerlings can barely be considered sentient.  Their language is one of grunts and clicks, completely incomprehensible and unreproducible to most humanoids.  They know how to do basic tasks like maintaining and using firearms, but other than that, all they do is eat, sleep, breed at a massive rate, and continue to dig out their massive fiefdoms under the ground.  Bunkerlings organize themselves into packs of 2-200 controlled by an alpha, a bunkerling that retains a shred of the cunning of its ancestors.  Under an alpha's command, bunkerlings are just as aggressive and unpredictable, but have a vastly-expanded repertoire of tactics to attack their prey.  Whenever a pack gets too large, another alpha is born, who leads exactly half of the pack away to start a new one.

Bunkerlings rarely come into contact with other races, unless they happen to share the same space in the Underdark.  For the most part, they hunt and scavenge peacefully, but when they encounter other sentient creatures, they shift into a sort of trophy-seeking behavior, attacking the creature for whatever interests them.  Pack alphas are notable for wearing the flayed skins of victims as cloaks, with their fingerbones strung onto necklaces.  The smartest alphas can understand the concept of slaves, and keep several forced to fawn constantly over the monster, and the alpha finds its protection from dumber ones of its kind attempting to eat them to be adequate compensation for their service.