Well in that case, I'm bringing this back up because I feel it would be quite the thing in those caves, though it would be basically worthless in fire-world
Design: Grue
Grue, an old word, to shiver or shudder from fear or cold. It fits the darkness, and indeed fits this idea we have conceived, these monsters, for they cannot truly be called anything else. The Grue are creatures formed from the etching darkness, the purest form of our masters cold, consuming grip, stabilized and solidified into a form that is, in shape, not too dissimilar from an ape, with a head taken up almost entirely by a mouth of ferocious fangs, and long forelegs that function as arms with feet that function as hands. The Grues most dangerous trait is its own flesh and blood, the darkness that makes it up may be stabilized, but it is not harmless, its skin burns to the touch, its teeth leak the etching darkness like a venom, and the bile in its makeshift stomach is also etching darkness, pure and unaltered. The Grue do not breath and thus make little noise, the Grue step softly so as to not be heard, the Grue move with purpose so as to never be noticed, by the time you notice a Grue, it is already upon you.
The Grue is not without its flaws however, to a degree they are by design. They cannot wield our equipment for any weapon, tool, or armor will slowly crumble away to their touch. Additionally, light is the Grues bane, too much indirect light, artificial or otherwise, repulses them, direct light burns their skin like pressing upon a wound, concentrated light tears their bodies apart. This makes them vulnerable, but means that the Grue are unwaveringly opposed to our enemy above all else, for the light is antithetical to their very being.
going to take a bit to come up with some good ideas, problem is that most of the obvious stuff to make for these planes is probably going to be the stuff we get from conquering these planes