Gibberling

Out of the moonlit dark rolls a seething mass of fur and pale flesh, screaming and chattering, dozens or hundreds strong. Closer, it becomes naked, hunchbacked humanoids with pointed doglike ears, black manes around leering faces and black eyes bright with madness, loping ever faster with short swords in overlong arms. Their killing done, they drift back into the night as randomly as they came.

The horde's howls, clicks, shrieks and insane jabber force even hardened hirelings to check morale every round; player characters check only if it fits them. There is no formation, only a flood moving in no set direction. Long practice gives them +1 to hit with their plain swords. They stop only to kill whatever moves, always fight to the death, eat all food in their path including their own fallen, and wreck any unfortified building or object.

The only real hope is their dread of bright light, and of fire most of all. A lone torch would be overrun, but a big bonfire or strong magical light holds them back or bends their course.

Their society, if any, resembles lemmings plunging into the sea or piranha in a frenzy: no sense, order or individuality. They somehow signal one another but have no discernible language. Captives babble only their own noise, refuse to learn anything else and fling themselves at bars and doors trying to escape. How, or whether, they reproduce is unknown.

On the surface they burrow in to hide by day and are then most vulnerable. Their trail of ruin is easy to follow, and dormant gibberlings can be dug up and dispatched; once uncovered they wake but mostly cower in the daylight. Underground they burrow or just curl up, and either way they wake suddenly together and erupt from the earth shrieking.

Every search for their lairs ends deep in caverns, where the trail fades on bare rock. Their manic lives demand vast amounts of food, and they strip any catch to the bone. Their fur teems with lice, their hides are worthless, and they carry no treasure, only pitted, dull, unmarked swords. They serve no master and no purpose but random death in the dark.