A peaceful tribe of nomadic wood-elves suddenly becomes hostile and starts raiding human settlements with murderous zeal. Investigating this sudden change of heart, the heroes discover that the tribe has abandoned its traditional nomadic lifestyle to take up residence inside a famous feature of the local landscape, the Hollow Mountain.
The Hollow Mountain is a very sharp, lone mountain that sticks up from the dense forests of the region like a rotten tooth. It derives its name from the fact that it is almost completely empty inside.
On their way to retaliate against the elves and rescue the prisoners, the PCs learn that plants and sylvan creatures in the area are being corrupted by some vile influence that must be destroyed before the whole forest turns on its inhabitants. As they proceed closer to the Hollow Mountain, the PCs encounter strange and hostile creatures; something is indeed terribly amiss.
Once inside the mountain, the PCs must brave cave after cave of different misshapen enemies, mutant elves and their animalistic masters, prehistoric plants and monsters until, at the top level, they pass into a nightmarish planar pocket through a portal opened ages ago by the highly-advanced lizardfolk who once occupied the hollow mountain and experimented with powerful magic that proved to be their undoing.
There they face the unlikely master of this insane unholy war, “the tree that sees”, a tree-like aberration from before the dawn of time that threatens to spill its madness into the PCs’ world.