Legacy

Legacy, Sword of Fleeting Godhood
Arachodus-Umbrite composite longsword created using several hundred live sacrifices. Known to drift throughout the Darklit cities, a great status symbol to the individual who possesses it.
Holding the sword grants the wielder crawler-like regeneration. They become capable of draining energy from the ambient surroundings to fuel healing, allowing them to shrug off significant amounts of damage. Slit throats, puncture wounds, and the like would barely inconvenience the wielder.
The unfortunate side-effect of this healing is that sufficiently structurally significant damage will heal imperfectly: if an entire limb is chopped off, there is no guarantee that its replacement will be entirely human. The replacement would then rely on the sword to sustain itself, rapidly draining the user's own magical reserves if they release said blade, and likely killing them (especially in the case of Cotd with their low energy reserves). Many a bearer simply fell apart once the blade was wrested from their grasp.
A secondary, lesser-known phenomenon is that the mass drawn upon to regenerate larger wounds relies on a link to the plane of darkness. If one were to lose, say, an arm, while within the Celestial claim of a deity, the blade would be unable to regenerate said arm: though the wound would close, the supply of additional body mass is cut off, so nothing is regrown.

The common story

Originally a disappointment, the blade was eventually used to kill the Shrouded who crafted it. In doing so, the weapon is said to have inadvertedly absorbed fragments of the deceased's crawler-infused soul. This is what turned a perfectly sharp, yet mundane, blade into a feared artifact.

Construction

The blade of spidertooth fiber drawn out to be near-invisible and woven around a complex frame to form the base shape on a 2D plane, making an exceedingly sharp edge (a single, extra-thin 'thread' in thickness). This is then set onto a thin metal 'core' section, ground to meet the edge cleanly (although there's around 4mm of '2D plane' extruding from the smooth transition between metal and spidertooth), used to add weight to the blade. The core then had fibers of spidertooth wound into it in an intricate geometric pattern both for security and aesthetics.
The crossguard is a relatively simple straight design, though with the edge on the side of the blade also being umbrite-reinforced. The center of the blade where it meets the crossguard is hollow, a combination of dick waving (fuck you, engineering practices!) and adjustment of the center of percussion. Mostly the former, considering how little the blade weighs.

Celestial

More importantly than its ludicrously expensive physical construction is its magical one. Its creator didn't sacrifice Cotd to draw out their Will: that'd be extremely inefficient, given that humans from other regions have far more capacity. Instead, he specifically used Cotd because their already mutilated souls are more malleable and susceptible to corruption. Legacy is Celestially constructed of several stitched together second-order soul scaffoldings, artificially replicating Artifact status (as opposed to the 'natural' way of accomplishing great deeds etc.). Over the centuries, this construction has been smoothed by time such that it's functionally identical to a naturally generated artifact in external signature.

Abilities

The blade grants crawler-regeneration and has Celestial Significance to match most Shrouded. It gets around the 10-micrometer rule by immersing the entire 'edge zone' in umbrite, with the actual cutting edge being near-mono-molecular spidertooth. Ordinarily this would be exceedingly energy-expensive to maintain, but the energy-vacuum effect of crawler souls keeps the edge rather coherent, preventing the leaking shadow from dispersing. Striking hard surfaces does create 'sparks' of shadow, but these are more a visual effect than anything. This edge is hard & sharp enough to literally shave off ribbons of hardened steel if it ever gets into a bind against an ordinary blade, and would likely shear through typical (low carbon) steel if met head-on.
Due to not being a true soul graft, wielding the weapon doesn't grant any soul replenishment. It accumulates its own soul energy instead, though this is relatively minor, taking a few seconds to refill the meager quantity. Nevertheless, it can be used as an arcanism aid. The blade is capable of projecting itself with solid shadow using this energy, extending reach.
Legacy doesn't really multiply the power of a Shrouded very much. It can up the regen factor and give them a bit more soul energy to draw from, but overall doesn't bump them up an order of magnitude due to them already having crawler souls. If anything, magical signature dissonance from the two crawler soul fragments can potentially reduce their regen factor.
This rarely comes up due to its bearers historically being Cotd, but Legacy also confers agelessness. It can't quite reverse the effects of existing aging, as its healing is imperfect in the first place, but the more severe issues such as ruined joints or blindness would be (mostly) fixed. This part of the blade's abilities are far more emphasized in non-Cotd cultures where old age actually exists.
Prolonged possession of the sword will subconsciously teach the wielder how to utilize its abilities.

As a sword

Note that Legacy is absurdly light, something in the realm of ~200g. This lack of mass makes it extremely poor at blocking hits. Blocking edge-on can theoretically present another whole set of problems with the severed weapon portion continuing to the defender's body at full speed. Parries fare better by virtue of being more about deflection and redirection than absorbing energy, though they're still harder than with a more conventional sword.
This also makes it rather poor when attacking very hard mundane metals. While infinitely durable, the edge only has mundane sharpness on its side (unlike something like an Epacsi greatsword, which uses energy haze to 'cut'). It won't cut through plate armor because its wielder can't generate the force to do so- the blade's sharpness isn't the bottleneck here, and the weapon lacks mass.