What do you think is in there?

The abandoned laboratory of an extinct culture, a place where something went very wrong, something went kinda right, and something got out. That third something gorged on local livestock, so the valley’s farmers pooled their resources and are offering you a heifer to enter the creature’s presumed lair and dispatch it. But the village oracle, between huffs of dream smoke, declares any who enter and live will be forever changed, then mutters something about extra limbs…