After being rescued from Cragmaw Castle, Gundren Rockseeker asked the Heroes of Phandalin to help him regain control of the mine in Wave Echo Cave. In exchange, he offered the party a 10% share of the mine's profits in perpetuity.
In the cave's entryway, the party discovered the remains of Tharden Rockseeker and helped themselves to his Boots of Striding and Springing. As they delved deeper into the dungeon, they battled ghasts, stirges, and slimes that wandered the caves. At some point, the party found a Wand of Magic Missiles clutched by a skeleton in a pond.
The heroes discovered the Forge of Spells, and convinced the centuries-bound spectator that guarded it that its services were no longer needed, at which point the spectator promptly vanished. After searching the Forge of Spells, the party claimed Unknown and Unknown from the Forge's workshop, and doused their weapons and armor in the Forge's green flame for a temporary power boost. They then met Mormesk, the last remaining wizard defender of Wave Echo Cave. Upon learning that the party had "defeated" the spectator, Mormesk's soul found peace and he faded away. In the central smelting cavern, the party encountered a flameskull tasked with defending the mine, but it and its skeletal minions were no match for the heroes.
They eventually encountered a dark elf, the same one that they killed at Cragmaw Castle,
leading a group of bugbears excavating a section of the mine. They
defeated the bugbears and dark elf, which was revealed to be another
doppelganger working for the Black Spider.
The heroes came across Nundro Rockseeker, who was bound and unconscious behind a locked door. They opted to leave him be for his own safety until they had confronted the Black Spider. A bit further past that room, they came face to face with the Black Spider herself in an ancient temple to the dwarven god Dumathoin. They battled the drow wizard, her giant spider pets, and her remaining bugbear guards. The heroes defeated all who stood in their way after a hard fought battle. As they inspected the temple, the Black Spider's unconscious body transformed into a fearsome drider, but was eventually killed as well. The group claimed her Unknown, and found a Letter to the Black Spider at her campsite.
The group returned to and healed Nundro, who was appreciative for his rescue and news of his brothers. Nundro led the party back to the dig site and revealed the location of a pair of Unknown. Having successfully cleared the mines of its deadly inhabitants, and finally foiling the Black Spider's plot, the party returned to Phandalin with Nundro. The town held a massive feast to celebrate the newly dubbed Heroes of Phandalin and their accomplishments.