A night hag and former concubine of Geryon, Liabara the Frigid, implores the PCs to take a Stygian version of the ice bucket challenge, a way of raising awareness of the plight of her lover Geryon, who paid a heavy price for his loyalty to Asmodeus. (The ice bucket is huge and contains an array of sword-like icicles. Treat as a cone of cold that inflicts cold and piercing damage, "cast" at a sufficiently high level to threaten your PCs.) This is a good opportunity to reveal some backstory on Geryon, the plane, or other hellish matters of note that may serve the PCs later in the adventure.
Liabara offers the PCs a choice: Offer a reasonable amount of gems, jewelry, or objet d'art as a sacrifice to Geryon or take the ice bucket challenge. In exchange for either the treasure or taking up the challenge, the night hag can help the PCs get to a particular destination faster by allowing them to travel magically through the twisted, hopeless dreams of Stygian petitioners.
If the PCs refuse Liabara's offer or attack her, she withdraws to the Ethereal Plane and plots her revenge. At least one of the PCs will have trouble sleeping during subsequent long rests as the night hag's protracted campaign of nightmare haunting begins.