After being rescued from Cragmaw Castle, Gundren Rockseeker asked the players 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.
They eventually encountered a dark elf 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. At the dig site, the party discovered a pair of Gauntlets of Ogre Power.
The party ventured deeper into the cave, where they came face to face with the Black Spider herself in an ancient temple to the dwarven god Dumathoin. They battled the dark elf wizard and her giant spiders, eventually knocking her unconscious. While the party finished off the spiders, the Black Spider transformed into a drider and attacked, but was eventually killed as well. The group claimed her Spider Staff, and found a Letter to the Black Spider and a key at her campsite.
The key opened a nearby door, behind which Nundro Rockseeker was found bound and unconscious. The group healed Nundro, who was appreciative for his rescue and news of his brothers. The dwarf accompanied the party throughout the rest of the dungeon while doing his best to stay out of harm's way. Finally, the group battled a spectator that was conjured to guard the Forge of Spells, and reclaimed it for the Rockseekers. For themselves, they took Lightbringer and Dragonguard from the Forge's workshop with no objections from Nundro.
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.