Frank Belknap Long was Lovecraft's close friend, and with this one story he added a shape to the Mythos that nobody has been able to remove. Halpin Chalmers takes a drug that lets his mind travel backward along the whole length of time. He goes too far. He learns that time has two kinds of geometry. Pure, clean things keep to the curves. The other things, the ancient and the foul, move only through the angles. And once they have your scent across the angles, they can come.
Chalmers boards up his study and plasters the corners round, because a right angle is a door and he knows it now. It does not save him. The genius of the story is that it makes a corner frightening. You finish it and you glance, just once, at where your own walls meet.
This is cosmic horror done as mathematics rather than as brooding dread, and it points forward to a whole lineage of weird fiction where the threat is not a monster but a property of space itself. Long took Lovecraft's vast indifferent cosmos and gave it teeth that live in the one place you cannot wall off.
Heard on Cyclopean Frequencies, where the Hound runs through the record. Listen on Bandcamp.