The oddly-matched travelers huffed and puffed through the dust of the Underdark bickering at their campsites until they finally fell asleep. The sleep did not calm the low hum of stress, however, because through the night, the party would be stung awake by the constant bites of insects. This, eventually replaced by horrid dreams of festering wounds exploding into clusters of spiders.
One dream, in particular, stood out to the adventurers of a torso of a giant spider demon with chains instead of its eight legs attached to hooks that were impaled into the party's backs, so that they were all tethered in agony to the beast.
They all wake in shock and see a formation of bites in the shape of a mysterious symbol on their arms…