The Hounds of Tindalos

Halpin Chalmers takes a drug to send his mind back through time, and in doing so draws the attention of the Hounds, lean things that live in the angles of time while all clean life keeps to its curves. Having been noticed, he cannot be unnoticed. The Hounds follow him home through every corner and angle of his room, and no plaster curve he builds can keep them from the joints of the world.

Long's story added a lasting idea to the Mythos, that geometry itself can be a door, and that some predators hunt not through space but through the sharp places where time bends. It is the origin of the Hounds that have prowled weird fiction ever since. The Archive runs it as a chase, a blues for the thing that comes through the angles.