Publisher's blurb: Manistrad Copperlocks and her dwarven crew have stumbled upon a secret beneath the mountain. The dwarven miners have broken through into a underground river, but that's not what worries them. It's where it leads that has them concerned. They need a brave set of souls to investigate and see what is in store for Saltmarsh. The adventurers venture deep into the underdark below Saltmarsh and uncover a dastardly yuan-ti plot. This adventure is presented as APL 10 (adjustable) and is intended to fill a level gap left in the Ghosts of Saltmarsh hardcover book. It ties into various Scarlet Brotherhood schemes around town and makes use of locations throughout Saltmarsh, such as the mine, the Wicker Goat, the Standing Stones, and more. This adventure takes 8 to 12 hours to play through.