Though grateful for his rescue, Sildar urged the party to focus on rescuing Gundren from the Cragmaws, who had taken him to their main base of operations, Cragmaw Castle. The party's first order of business was to locate the castle. Qeline Alderleaf, a halfing farmer in Phandalin, mentioned that Reidoth the druid knows the area very well, and that he was last seen heading to the Ruins of Thundertree.
The party ventured to Thundertree, where they encountered a young green dragon named Venomfang posing as Reidoth. Venomfang claimed that a group of dragon cultists had stolen two roc eggs, mistakenly believing them to be dragon eggs that they could hatch and raise into servitude of their cult. These eggs were in fact Venomfang's, who wanted them back at any cost. The party attempted to negotiate with the cultists on behalf of "Reidoth," though their efforts were unsuccessful. Lungra smashed one of the eggs, which enraged Venomfang and caused her to attack in her dragon form. Venomfang instantly killed all of the cultists with her poison breath, and then attacked Lungra for destroying her egg. After a fierce battle, the party managed to defeat the young dragon. The real Reidoth was discovered bound in a chest in one of the ruined buildings. Grateful for his rescue, he agreed to lead the party to Cragmaw Castle. As they parted ways, Reidoth took the remaining dragon egg with him, vowing to raise the dragon as a force for Good.
The party scouted the castle's defenses before stealthily infiltrating it to rescue Gundren. They were able to bypass most of these defenses, and found a direct path to King Grol. The bugbear king was in the middle of intense negotiations with a female dark elf, with Gundren's fate in the balance. The party ambushed Grol, his pet wolf, and the dark elf by blasting them with their scroll of Lightning Bolt, initiating combat. During the fight, Grol cracked open his "boomstick" which caused a thunderclap to ring throughout Cragmaw Castle, drawing the rest of its defenders into the fray. The party fiercely battled against waves of goblins and hobgoblins, and eventually Cragmaw Castle fell silent. When the dark elf was slain, her form morphed into that of a gray, featureless humanoid, which the party identified as a doppelganger.
After defeating the Cragmaws, the party healed Gundren's wounds and restored him to consciousness. He expressed his gratitude, and asked the party to escort him to Phandalin so they could begin planning the recovery of Wave Echo Cave.