From the publishers website:
The Hollow Mountain
The Hollow Mountain is designed for four to six characters of 8th to 12th levels. We recommend 40–50 total character levels between the party members. While the characters can be of any basic character class, a good mix of character classes is helpful. A rogue, strong warrior type, and good-aligned cleric are all extremely valuable in this adventure, druids will be especially useful in the Hollow Mountain and have the greatest motivation to participate in it. A traditionally peaceful tribe of nomadic wood elves suddenly becomes hostile and starts raiding neighboring human settlements with murderous zeal. It seems that the main object of these raids is prisoner capturing and wanton, senseless destruction of any man-made objects.
The PCs are hired by the local baron to investigate the cause of this sudden change of heart in his former allies and soon discover that tribe has abandoned its traditional nomadic lifestyle and taken up residence inside a famous local landmark called the Hollow Mountain.
The Hollow Mountain is a very sharp, lone mountain that sticks 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 rescue the victims, the PCs encounter a wounded and scared satyr who tells them that plants and sylvan creatures in the area are corrupted by some vile infl uence that must be destroyed before the whole forest turns on its inhabitants. As they proceed the PCs are attacked by strange and hostile plant creatures that resemble man-sized flowers with dagger-like petals and groups of elven fanatics armed with poisonous arrows. These fanatics claim to be leading a holy war against the unnatural and corrupted civilization and the “false” that produced it.
Once inside the mountain the PCs must brave cave after cave of different misshapen enemies, mutant elves and their animalistic masters and prehistoric plants and monsters. Finally, they pass into a nightmarish plane through a portal opened ages ago by highly-advanced lizardfolk who once occupied the Hollow Mountain and experimented with powerful magic in its higher levels. 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 world.