Could these demons exist?

Question

The demons are large subterranean humanoids. They are human-like in form and intelligence, and around 8ft tall when stood up. Their face is elongated, with a vaguely canine-appearing snout. They have no eyes, and have multiple stiff quills on their forehead, with the arrangement varying between individuals. They also have beards and head-hair. Their ears are pointed, somewhat like a monkey. Their arms are longer, and they can move on all fours with the same ease as on two legs. Instead of fingernails and toenails, they have what look to be claws. They also have a tail, which is tipped with a set of smaller quills

Their habitat is the underworld, which is a strange set of caves. The underworld is highly radioactive, and quite hot (60°C). It is inhabited by strange, vine-like plants that have adapted to use the radiation instead of sunlight. These are the producers in the ecosystems, and provide food for many animals that dwell in the underworld. The demons, while they originate in the underworld, are capable of surviving and adapting to the Earth-like conditions of the surface world

Do these demons make sense, and could they exist?

Answer 1

Your ecosystem needs fleshing out

Using radioactivity to power an ecosystem is iffy - it's hard to suppose the amount of radioactivity the genetic code can withstand will be enough to power an organism directly. One option is to use an intermediate chemical, such as hydrogen gas produced by radiolysis of water, so that radiation over a vast subterranean volume merely gives rise to vented fuel. Your plants could take up carbon dioxide and hydrogen and convert it into carbohydrates, while your "demons" might cluster near downdrafts of oxygen, and venture further into the tunnels on stored oxygen like marine mammals.

Temperature is an issue: all the enzymes have to be optimized to fold properly and function at a wide range of temperatures unless the demons maintain a high body temperature topside.

The bristles are presumably a tactile sense, and the lack of eyes suggests there isn't much bioluminescence in the caves. The caves are clearly very large, with some unusual geology. Considering the temperature, it is a pity these animals didn't evolve near IR sensation akin to pit vipers at least. The claws are not easy to explain - there must be substantial sediment to dig through in the caves; it's hard to picture your vines are a prey that requires talons to catch. Maybe they evolved for obtaining supplemental nitrogen and phosphorus from unwary townspeople on the surface.

Answer 2

Could they exist? Yes.

Would someone tall and lanky, like me, want to live in a cave where I’m forever banging my head and slouching? Never. I would hope evolution steps in to make creatures like most other subterranean animals; horizontal (i.e. naked mole rats, moles, worms, voles, etc.)