After Edgar Allen Poe, but before Stephen King, H P Lovecraft took the horror genre and mixed it with sci-fi, mythology and folklore. One of the best horror writers, period.

I highly recommend reading:

Herbert West-Reanimator
The Call of Cthulhu
The Dream-Quest of Unknown Kadath
The Colour Out of Space
The Dunwich Horror
The Whisperer in Darkness
At the Mountains of Madness
The Shadow Over Innsmouth
The Dreams in the Witch House
The Thing on the Doorstep
The Shadow out of Time
The Haunter of the Dark

I need to reread some of his other stories, but the ones I have mentioned have stuck with me for years.