From Introduction:
HP. Lovecraft’s “Dreams in the Witch House” describes a disturbing series of encounters between a theoretical mathematician, a furtive ratlike creature named “Brown Jenkin,” and what very well might be the spirit of a 17th-century sorceress. Jenkin served as the familiar of Keziah Mason, a cunning old woman who escaped Arkham’s witch trials by opening a gate to another dimension. Centuries later, Walter Gilman used mathematical acumen and lucid dreaming to access this forgotten realm. By his reckless investigation, he released an ancient horror into his moldering garret apartment inside the now infamous “Witch House.”
In May of 1931, most of the Witch House was destroyed by a terrible storm, but the fate of Keziah Mason remains a point of some conjecture. Workers sorting through the rubble found the gnawed bodies of several children who had disappeared from Arkham over the years, along with the corpse of a twisted, ratlike thing, and the bent and desiccated remains of an elderly woman.
Many occultists and investigators perusing reports on the Witch House have concluded that the withered old body was indeed the remains of Keziah Mason. Yet a few harbor doubts. What if the corpse wasn’t the old witch, but one of her victims? Or even worse, what if the corpse had been preserved in that state for centuries… even after the spirit of Keziah Mason passed into an otherworldly dimension bordering on our own? Was her familiar, Brown Jenkin, a unique anomaly, or is it possible that the beldame has since instructed other hyperintelligent creatures in her dark arts?
For enthusiasts of the macabre, this web enhancement presents an “adventure fragment” based on these possibilities. Like similar fragments for the original Call of Cthulhu Roleplaying Game, it contains the outline of a brief story, a handful of encounters, and guidelines for the various “crunchy bits” of the rules. This fragment, however, adapts the game mechanics for the d20 system. With a minimal amount of preparation and a copy of the Call of Cthulhu d20 Roleplaying Game, a Gamemaster can use this fragment to prepare anything from a short introductory adventure playable in one evening to the foundation for a tireless crusade against an ancient and insidious evil. This bonus material is an exclusive feature of the Wizards of the Coast website.
The first half of this story, intended for four or five 1st-level investigators, should be playable in a single evening. It’s a simple, straightforward investigation in which most of the characters (this time, at least) should survive without much difficulty. The body count is minimal, but the implications are horrendous. The second half not only details the consequences of that investigation, but also offers a possible foundation for an ongoing campaign. Some of the particulars concerning rat-things could be used in other adventures as well.