Haunt

Ethereal, semi-amorphous transparent spirits lined with hard light symbols that appear in the northern taigas during the night. Unknown to the locals, they are mostly extradimensional, and appear due to the North's thin dimensional boundaries with the plane of dark.
Typically bipedal in appearance, with sharp-tipped tentacles sprouting from various points. These tentacles can pierce through flesh with relative ease, though they're not as tough as ivory.
Extremely averse to sunlight. Common knowledge says that this kills them, but technically, the sunlight simply strengthens the barrier between planes and forces them to fade away. And then they starve, which kills them.
Haunts are just echos and shavings of the much larger things that drift in the Plane of Darkness. As a result, they're typically driven by instinct to consume in the desperate hope of amassing enough Celestial Weight to not be subsumed the greater gestalt, leading to their horrible reputation. Directed light can kill a haunt by selectively phasing them out, cutting them into pieces.
More developed haunts often have visible fragments of a skeletal structure embedded within their translucent bodies, along with external plating of varying size- from small scales to large plates of armor. Both of these tend to be made of the same oily, iridescent material closely resembling crawler scales.
Notably, haunts are not aggressive towards natives of the The Pitch Black Forest.
A haunt is essentially a particularly developed echo grown to inhabit the mundane plane- as a result, a Cotd sorcerer could theoretically create something very similar to act as minions.
An interesting quirk of haunts is that they generally don't kill sleeping people: due to dreams weakening the Mind-Soul Interface , the haunt instead feeds upon this dissonance reminiscent of how nightmares do. This is very niche (part of why full nightmares are so rare), seeing as they become aggressive once the person wakes. It's why unaware campers in the forests tend to be set upon as they wake up, rather than being attacked in their sleep.
A single kill on an average person can sustain a haunt for over a week. A haunt killing say, a war hero, can cause ascension then and there. There's a reason it's not a northern tradition to wander into the darkness and die if a harsh winter is expected, and it's not sentimentality.

Relationship with Darkness

Haunts are almost like a fungus or bacteria pervasive throughout Darkness, tangible manifestations being instances where the circumstances are favorable for the pathogen to grow. They're tiny seeds of potential, scavengers which consume the (metaphorical) rotting flesh of dead things within Darkness.

Life Cycle

Haunts typically fade in and out of existence, barely sentient echoes of greater things. Each passing night generates a whole new set of haunts, which return to the greater gestalt of the titan corpse over time. Stronger, more significant haunts can resist this for a while, but being erased is a constant risk unless the entity manages to fully transcend its limitations. In a way, haunts represent potential. They can grow into something more by gaining enough coherence, typically through Celestial significance. The shortest path to this, as most know, is snuffing out souls. Sustenance through slaughter results in the birth of a crawler. Even more rarely, Significance through whispered words and hushed stories creates a Nightmare. For a haunt to truly ascend is a rare thing- happening perhaps a handful of times per decade. Even then, they're not out of the woods. In the earlier stages of their life, it is entirely feasible for a group of mages and soldiers to deal with one: the strength of crawlers and nightmares is their nigh-limitless potential for growth. While a millennia-old crawler may be nigh unstoppable, most archmages could comfortably deal with one that has only lived for a century. Less than a decade old, and a particularly determined village stands a chance, especially if the crawler hasn't gained its own soul yet. Think of a haunts as an extremely prolific mold with spores scattered throughout Darkness, growing when sufficient levels of Will are dispersed into the environment.
Direct observation is generally enough to prevent haunt coalescence- they only appear in space not being visually observed. The other senses, however, do not afford protection. Should a haunt turn upon its brethren, it becomes a fratriphage. This is a dead end, restricting the haunt's potential for future growth. At the same time, it is the safest option for ensuring continued existence in the short run.
Fundamentally, the theme of haunts is desperation. From birth, the only thing they really know is that they will be consumed if they do not make enough of a mark on the world. It shows in their combat- minimal self-preservation other than avoiding blatantly deadly strikes. Why would you be worried about your arm getting cut off when the alternative is being erased from existence?
A haunt's end goal can be thought of as to gain a first-order soul:

Haunt to Crawler

In transitioning to a crawler , the haunt attempts to build up sufficient Celestial mass to coalesce a first-order soul. While possible, this is unlikely: a much more likely outcome is that one of the many people that the pseudo-crawler kills has a soul signature close enough to its existing amalgamation of values as to neatly fit. Until the nascent crawler finds such a person, their ambient energy drain is far weaker: barely sustaining existence past the night, not enough to exhibit the immense regeneration famous to crawlers.

to Fratriphage

The fratriphage transformation is a weaker version of this: because the being primarily hunts other haunts, the only way it could possibly gain a first-order soul (increasing its maximum Significance density massively) is for one to incarnate into it: this can take decades to centuries, and the creature itself generally dies long before then. That being said, there are exceptional cases where the gamble pays off, and a fully-souled fratiphage is born.

Haunt to Nightmare

On the other hand, a nightmare chooses to act like a parasite: while a crawler aims to take another soul into itself, the nightmare wishes to graft itself onto another. Once the nightmare gains a certain amount of strength, a suitable host can almost completely imprint their desire upon it (said desires having been twisted by the nightmare beforehand). This is part of why nightmares are so rare- their existence is somewhat paraoxical: surviving beyond being a haunt is a step towards individuality, yet giving another full control essentially strips that away.