The Troglodytes

Troglodytes are a mix of human and reptile. They are found in the Underdark, and usually in the region of the Massif Central in Frankia. There they have grown and spread out through the systems created by erosion on the ancient volcanoes & mountains that make up the region. They have existed for millennia, and seem to be mutated from Neanderthals who originally dwelt in the region. The Neanderthal ancestors seem to have been drawn to the caves and eventually delved too deep. Some ancient evil from the dawn of the world awoke to their presence and proceeded to warp them to it's own image. By doing so, it stripped them of their intelligence, and let them grow in their own filth. The resulting creatures are attuned to the darkness they now find themselves in with a hunger for flesh.
Exactly what this god was, or is if it still exists, is unknown. This is in part to the fact that Troglodytes speak their own language that no-one understands. They do draw the odd pictogram in blood and dung that seems to show a massive obese reptilian being sitting on a heap of broken bones and skulls. A few sites around the Massif Central shows Neanderthal cave paintings involving cannibalism and the odd one of this being.
Sometimes, the paintings show the god standing with it's arms wide, broken bones piercing it's wrists, and it's abdomen ripped open and it's intestines being pulled out with vines or creepers with bones puncturing both the ends & intestines.
Thankfully for the world they seem to only exist under the Massif Central at present, though the size of the Underdark, there is the risk of them spreading further into other territories if left unchecked.