Walter Gilman, a student of mathematics at Miskatonic University, rents an attic room in a house that is rumored to be cursed and haunted by the spirit of a witch, Keziah Mason, who lived there in the 17th century. He experiences nocturnal visions of Mason, her familiar (a rat-like creature called Brown Jenkin), and strange creatures in otherworldly dimensions, and if offered knowledge beyond the reach of human science, but at a terrible cost.
Loosely used as the basis for the film Curse of the Crimson Altar.
This story contains examples of:
- Acid-Trip Dimension: The dimension outside "angled space" (the 3-dimensional universe). It's a black space filled with portals, and living beings passing through it appear as strange shapes.
- Alien Geometries: Understanding these is the key to accessing interdimensional travel. The price paid for such passage, however...
- Bizarrchitecture: The witch house is built with Alien Geometries and has a door that somehow leans both to the left and right at the same time.
- Broke Your Arm Punching Out Cthulhu: Gilman manages to stop a servant of an Eldritch Abomination, but is unable to save the child from sacrifice. Though he manages to stay sane throughout the ordeal, he is later killed by Brown Jenkin drilling itself out of his body as punishment, and his friend, seeing this, ends up in a mental institution.
- Cameo: Elder Things from At the Mountains of Madness make a brief appearance during Gilman's unwilling interdimensional voyages, but don't otherwise influence the story.
- Deal with the Devil: Nyarlathotep appears to the protagonist, offering him complete control over the ability to travel outside the angled space (effectively being able make a personal wormhole between any two locations) in exchange for signing the book of Azathoth with his blood. The protagonist refuses, but judging from what kind of beings we're dealing with, it's probably better not to know what would've happened had he accepted the deal.
- Death of a Child: Gilman fails in his attempt to save a kidnapped child from the witch and her familiar.
- Doing In the Wizard: Gilman moves into the titular building because his curiosity is aroused by rumors that it is haunted by the ghost of the witch who once lived there. In a way it is, but it is heavily implied that the "haunting" is not actually supernatural but an extremely sophisticated science based around the manipulation of space and time.
- Familiar: The witch has a disturbing one, the human-faced, mean-spirited rat Brown Jenkin.
- Formulaic Magic: The protagonist is a mathematician who discovers an equation that would allow him to travel outside angled space (basically, to create wormholes).
- Haunted Headquarters: The house where the protagonist lives has a reputation as a haunted house but is actually periodically visited by a living, immortal witch who used to live in the house, and still uses its sealed attic for her work.
- Heterosexual Life-Partners: Walter Gilman and Frank Elwood.
- Human Sacrifice: The witch practices child sacrifice.
- Humanoid Abomination: The Black Man, who appears as a human male of indeterminate age with European facial features, but a night-black complexion.note Even apart from his unusual appearance, the narrator notices something off with him, and is quite right to do so.
- Keeper of Forbidden Knowledge: Nyarlathotep is characterized like this. He is a gatekeeper to secrets beyond human ken, and must be appeased with human sacrifices in order to relinquish some of this knowledge.
- Living Polyhedron: In one of his visions, Gilman has to make a pilgrimage to the Cold Waste beyond the stars to where the Old Ones dwell. On the way he meets many bizarre and eldritch life-forms, polyhedra included.
- Mad Mathematician: A brilliant young mathematician moves into the witch house precisely because of hints that its most infamous former inhabitant of over two hundred years ago might have possessed certain mathematical knowledge — notably about the geometry of space and time — not yet actually (re)discovered by modern science. It does not end well, partly because it turns out that said witch is actually still alive and still as nasty as ever.
- Magic from Technology: Or, rather, Sufficiently Advanced Science. It's shown that the witches in fact use Sufficiently Advanced Mathematics to teleport immense distances and grant themselves near-immortality, but they still do it in the context of a religion, or possibly it just looks like a religion to outsiders — the pathways beyond the three-dimensional space are guarded by Nyarlathotep, the Crawling Chaos, and he expects annual human sacrifices in return for their use (possibly simply as a sign of cutting themselves off from the rest of humanity rather than any practical purpose, though this isn't elaborated).
- Protective Charm: A cheap metal crucifix was used as a basic charm, and its eventual use only caused a small window of reflexive panic to loosen a tight grip. The crucifix's chain was more potent, in that it was used to physically strangle the witch.
- Time Travel: Employed by Keziah Mason.
- We Are as Mayflies: Keziah Mason had learned how to travel to spaces beyond time, and remained unageing, save for the brief visits she made back to Earth once a year.
- Wicked Witch: Keziah Mason is a rather typical example of this trope being old and reclusive, using something like black magic, and making a deal with a Satan-like deity.