Distraught Spirit
the witness who begs you to stop it
HP — Morale — No armor, No attack
A once-per-scenario weird encounter inside the Accursed Den. Agitated by the depravities and cannibalism plaguing the Den, the spirit pleads with the PCs to end the horror. Can tell them about the Gutworm.
Special: No stat block is printed. The spirit does not fight, only pleads — it remembers something that was done to it here, but not its own name. Manifests in the Pump room, the Chain room, or the Debris room (the three rooms with random-encounter rolls).
Lore: Whatever the spirit was — a prisoner of the Den, a Courtier of the Shadow King gone looking for Aldon, a cannibal's leftover — it carries enough memory to point at the worm in the sludge and hate it more than it hates being dead. Once the PCs hear it out, it dissolves.
No HP, no Morale, no Special mechanics, no name — a thin-source entity rendered as a scenario hook rather than a creature you fight. The spirit's identity is the obvious expansion seed; tying it to a named NPC (perhaps the Seer who hired the PCs) would deepen the scenario considerably.