This is the follow-on adventure to Return to Phandalin, which provides an updated town several months after the events of the starter boxed adventure Lost Mines of Phandelver. In this module, the PCs are once again asked to help Gungren the dwarf in defending the mines.
Kuo-toa have come from the underground lake and attacked the mines. The PCs sail upon the lake and discover a demon prison that the fishmen have been worshiping and eventually explore their underground city that was once a dwarven stronghold.
For 9th - 10th level characters.