Ghost Cap

Stringy, luminescent fungus that grows in the Shattered Jungle. Their light has the unique property of completely ignoring (i.e. being completely perceptible through) Darklight. Distilled ghost cap is used to make ink by Abyss Dwellers.
Can be drawn into fibers to make Umbrite. Quite an expensive and time consuming method, so most Shattered Jungle umbrite is of the imperfect variety made from ordinary paper.
Interestingly, ghost cap-based torches could be used to explore the Pitch Black Forest even without the darklander's fancy eyes . It's rather unsettling, working with a single torch in an ocean of darkness, but it'd be possible. Some trade-focused Darklit cities with good relations to the Abyss actually import ghost cap to use as lighting, making their environment more palatable to outsiders.
Ghost caps' "light" is actually a form of illusion which interfaces directly with the souls that it touches. This is why it works through darklight- it isn't strictly light, so much as a sympathetic illusion which pushes the concept of light upon those within it. The illusion's limited range also explains why the light seems to cut off at a much shorter distance than expected. As a result, it doesn't pick up on any (theoretical, for the most part) cameras.
The complexity of the ghost cap illusion, especially once one considers that it interacts neatly with any number of light sources, is one of the holy grails of sorcery. Vhagar himself was unable to replicate the effect to infinite scalability without using some sort of dimensional refraction/linkage (considered cheating by sidestepping the actual problem).
Umbrite pieces can be spliced into ghost caps in order to increase their 'brightness', drawing energy from the solid shadow.

Cultural Perceptions

Despite historical associations, ghost caps aren't native to the Pitch Black Forest, and actually exhibit rather stunted growth when planted within them, as they rely heavily on the southern jungle's powerful magical currents. The fungus was first brought to the Darklit Cities through trade with Chasmers (linked directly to the cities by a Veil), who obtained it from Eidel. This continued being the only reliable source of it even long after Eidel's fall, as the fungus could only be effectively grown in one region on the continent.
Of course, to those unaware of this history, ghost caps seem quintessentially Cotd. Their extensive use in lighting most foreigner-minded districts cements the association for merchants and travelers. Meanwhile, despite being native to the region, Eidel very rarely did much with the plant- rabbit lamps were far less tempermental, and shadowroot provided a better and more easily-isolated organic Darkness-aligned substance.