A small town near Neverwinter that was destroyed by the eruption of Mount Hotenow in 1451 DR. The ruins were inhabited by ash zombies, malevolent spirits of those who couldn't escape the destruction, as well as twig blights that had flourished in the nutrient rich soil.
The druid Reidoth had recently arrived in Thundertree to drive off the green dragon Venomfang. Several members of the Cult of the Dragon had also come to the ruined town, seeking to parley Venomfang's aid in their goals. The cultists stole Venomfang's eggs, putting them in a delicate negotiating position. Venomfang had managed to capture Reidoth and impersonated him in order to recruit adventurers into slaying the cultists and returning her eggs.
The Heroes of Phandalin smashed one of the eggs, causing Venomfang to kill all the cultists with her breath weapon before falling to the heroes' might. The party rescued Reidoth, who led them from Thundertree to Cragmaw Castle.