The Dreams in the Witch House

"The Dreams in the Witch House" is a horror short story by American writer H. P. Lovecraft, part of the Cthulhu Mythos cycle. Written in January/February 1932, it was first published in the July 1933 issue of Weird Tales.
Walter Gilman, a student of mathematics and folklore at Miskatonic University, rents an attic room in the "Witch House", a house in Arkham, Massachusetts that is rumored to be cursed. The house once harboured Keziah Mason, an accused witch who disappeared mysteriously from a Salem jail in 1692. Gilman discovers that, for the better part of two centuries, many of the attic's occupants have died prematurely. The dimensions of Gilman's attic room are unusual and seem to conform to a kind of unearthly geometry. Gilman theorizes that the structure can enable travel from one plane or dimension to another.
Gilman begins experiencing bizarre dreams in which he seems to float without physical form through an otherworldly space of unearthly geometry and indescribable colors and sounds. Among the elements, both organic and inorganic, he perceives shapes that he innately recognizes as entities which appear and disappear instantaneously and at random. Several times, his dreaming-self encounters bizarre clusters of "iridescent, prolately spheroidal bubbles", as well as a rapidly changing polyhedral-figure, both of which appear sapient. Gilman also has nightly experiences involving Keziah and her rat-bodied, human-faced familiar, Brown Jenkin, which he believes are not dreams at all. In other dreams, Gilman is taken to a city of the "Elder Things" and even brings back evidence that he has actually been there-a miniature statue of an "Elder Thing" which he breaks off from a balustrade within the city. The statue is made of unknown materials and a strange kind of alloy...