This is the piece of work Lovecraft is perhaps most famous for. Games, Movies, Comics, Plushies, and authors worldwide have been inspired by this story. Even the author's legacy, "The Cthulhu Mythos," is named after this story.
Fun fact, though: Lovecraft himself referred to his connected writings (as a sort of jest) as "Yog Sothothery" after another of his entities.
The first part introduces us to our narrator and his initial discoveries of strange dreams, widespread pockets of madness, and a strange bas-relief that will lead him down a rabbit-hole of non-Euclidean structures, cults, and living nightmares.