The Vile Worm (S&W)

from the adventure background:
Deep within the forest, an ancient oak has grown huge, twisted, and evil. Ages ago, a savage cult haunted these woods and this tree became the focus of their unspeakable rites. Below it, they carved out a chamber of sacrificial horror where innocent victims were offered to a hideous worm-like god. As the centuries passed, the cult faded into the mists of time, but the twisted old oak stood fast, awaiting the day when the creeping evil in the dark below would be summoned once more.

from the introduction:
This module is a short, but challenging scenario for a party of low-level characters.