Possible sighting of a Shramgalder.
Connected to the surface via straight up-down mile-long tunnels are underground laboratories. The tunnels have no stairs/ladders/elevators and their sides are sheer stone. The laboratories are filled with vats of bobbing body parts, preserved in a stinking liquid. Any one who spelunks down the tunnel and makes it into the laboratories will find that the body parts bubble, shake and mutate constantly.
Attending these vats are Human Sized Bat-Snakes, known as the Shramgalders. In the laboratories they slither upright, tinkering with the globs of flesh with delicate hands at the ends of their wings. At night they fly out of their laboratories to harvest more flesh for the vats. The skin under their fur bubbles and morphs, a side effect from too long spent near the preserving-mutating liquid. The completed products of their flesh melding exercises are unleashed deeper into the underground. Shipments of the vat-liquid are supplied monthly at a shadowy cove, delivered by a group of mercenaries under the hire of a distant nation.