Crawler

Strange, twisted creatures that grow and learn through regeneration after taking damage. Natives to the Plane of Darkness, but one of the easiest entities to accidentally bait into coming through dimensional tears. Due to this, they are what most cultures think of when the plane of darkness is mentioned. This preconception feeds into the likelihood of attracting crawlers, so the cycle continues.
Exact appearance varies heavily on the individual, although opalescent, oily scales and long, segmented tentacles are common. They are one of the few darkness natives that normal humans can perfectly see the external features of, although their typical grotesqueness really doesn't help alleviate any fears.
Some Cotd believe that the abundance of haunts in the north can likely be attributed to a battle between titanic crawlers aeons ago. Even their split blood clings to the idea of regeneration and growing stronger, resulting in the infestation known in modern times. Or so the tale goes- it's rather hard to verify.
Crawlers' souls constantly feed on the ambient energy around them, leading to their surroundings often feeling duller and emptier. A crawler can be killed by damaging it enough to drain the area surrounding it it dry of energy for its soul to use. This has the unfortunate side-effect of reducing the surroundings into barren wasteland, possibly miles in radius depending on the size of the crawler. Coherent souls are much harder, though not impossible, to drain than ambient energy, so sentient creatures in the radius will be largely unaffected.
Note that crawlers grow across magical space along with mundane. Younger crawlers of seemingly similar size to elder ones may be orders of magnitude smaller, as the latter will extend to all local loose superimposition . This also renders older crawlers immune to many armor-piercing effects, as they tend to rely on either bypassing physical defense by slipping through loose superimposition, or being denser than the target. While not strictly Chaos-aligned, elder crawlers can create a vague halo of Chaos-effect, especially when moving through magically complex space, as their interdimensional bulk can cause stretches in local superimposition.

Krakens share a lot of soul-structure (and consequent energy-leeching abilities) with crawlers, but have much more coherent and consistent body structures.

Origins

Crawlers are have been described as "self-sustaining cancerous growths". They are an eventuality of Darkness allowed to run free without a mind to focus them. Haunts turn into crawlers because all they do is consume and grow- by the time they gain a soul to potentially direct them, they've already spent decades to centuries building up a soul structure entirely specialized in consumption.
Their ability to drain ambient energy is actually a side-effect of the main purpose: minimize internal entropy, letting as little of the 'crawler' escape and diffuse as possible. Somewhat ironically, this makes the sword Legacy a closer fit to the intended purpose than the Shrouded.