A giant mushroom that messes with your mind? Meh. In Golarion’s Darklands (or any of the WotC worlds’ Underdarks) I’m surprised when the mushrooms don’t mess with your mind. A mushroom that with ties to the Shadow Plane that seems to dim the light? Again, sure, why not? But a mushroom whose spores permanently alter any flame that is exposed to them, creating black flames that aid in hypnosis? That’s interesting. That’s as interesting as the halfling race’s Blackflame in the Known World/Mystara! I don't know what I think about it (or how the fantasy physics would even work) but the notion of Underdark races religiously tending to rare blight fires (the same way early man probably tended flames left over from lightning strikes) is really appealing. And the fact that xulgaths (the original, non-degenerate—so…just generate?—troglodytes) are immune to the spores writes you some adventure seeds right there.
Novice adventurers look for work with a caravan. They don’t have the experience to join the ’van proper, so they're left with the unenviable assignment of guarding a fortune-teller’s wagon. All goes well enough at first, with the adventurers and the fortune-teller getting each other out of one scrape or another along the road and in the small hamlets they stop at. Then disaster strikes: A hobgoblin raid leaves the wagon tipped on its side…and in the process, the fortune-teller’s carefully tended blight fire goes out. Fortunately the caves where she first discovered the strange fire are not far, and she gives the adventurers her last few magic items if they will help her find a new source of the flame. The subsequent spelunking takes them far deeper than they expected…which may be a good thing, since by the time they find the source of the blight fire (a tenebrous blight grove) the adventurers will be experienced enough to survive it.
A gnome studies a disease that has afflicted his race since their exile from the fey realms. He believes the color-leaching properties of the tenebrous’s blight’s psychic trauma spores may offer some clues—perhaps even an antidote. He taps adventurers to help him find the shadow fungi, only to discover a grove tended by dread gnomes (see the Advanced Race Guide). The dread gnomes attempt to coerce the party into joining them at a feast where tenebrous blight steaks are the main course…and the spores are unavoidable.
Whenever a certain svartalfar sets up a meet to discus business, he does so with a tenebrous blight on hand. He finds that customers who can stand up to the paranoia-inducing mushrooms tend to be reliable business partners, and anyway his xulgath second-in-command is immune.
—Occult Bestiary 55
Stats for the tenebrous blight don’t seem to be online yet, but you can always find them here.