Spectral Worm

Name: Spectral Worm
threat level: varies
size: 20-30ft

(one of two ideas I'm posting from discord to here)

The Spectral worm is a subterranean serpentine creature with an incredibly pale, almost translucent surface. The "skin" is appears to be some hybrid between a rough rocky pattern and a very leathery surface. The translucent appearance of the entire body is actually due to a bizarre, natural ability for the worm, which resides permanently phased partially in Lydus and in Felarya at the same time. Unlike the Negative Hydra, which resides fully in Lydus and reveals itself in Felarya, this creature resides partially in both, and fully in neither.

This is what allows it to sail through the ground without actually digging through dirt. It swims through ground it like a Mermaid in water. It follows the ley lines of the world and areas of high thermal energy in groups of other spectral worms. Most subterranean creatures leave them alone, because of the natural ability to float through solid objects makes them very difficult to consume.

This creature lives off of subterranean heat, absorbing it like a desert plant, and processing it inside their body, and releasing a gas that embeds itself into the ground it swims through. These areas become highly concentrated with magic as a result, and absorb into Felarya's ground, further amplifying the natural magic properties of the world.
It does have a mouth though, which it can use to trap a living creature within its body, but because it feeds off of heat, it only goes for warm blooded creatures if it ever "swims" close to the surface. It has grooves across its body which function similar to a normal creature's sensory perceptions, which is one of the ways it detects heat through the ground - and similarly any adventurer trying to delve into subterranean ruins. Anyone unfortunate enough to be caught in its "mouth" will be subject to a mind-numbingly long digestion, in which whatever part of their body isn't processed is expelled as that same "magic-gas". However, due to its habits, it rarely travels that close to the surface.
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tl;dr - it "eats" heat, shits out magic gas. it exists in a permanent "phased half way in half way out" state, and migrates in groups. usually doesn't eat people, but it can. Also doesn't go close to ground level too often, either.