Ok, maybe not a master of horror in the traditional sense, but he's definitely someone whose work has given me the heebie-jeebies, and someone who I'd like more people to know about.
Junji Ito is a Japanese horror manga artist born in 1963. His most recognizable works are the Uzumaki and Tomie manga series. His works often involve seemingly normal individuals driven to madness or worse by obsession.
My first experience with Ito's work was seeing a couple of the Tomie movies (based on the manga of the same name) during the height of the J-Horror craze in the early 2000s. In Tomie, a detective discovers a long string of unsolved murders in which all the victims share the name Tomie Kamikawa. Meanwhile, an amnesiac young woman slowly discovers her connection to another Tomie Kamikawa.
There have been 9 Tomie movies released between 1998 and 2011, several of which are available on YouTube. The most interesting thing about the films is their mythology; they aren't quite scary or strange enough to be particularly notable, but they are still good fun. Tomie: Replay is one of the better installments (unfortunately the sound synch is a bit off in the linked video).
A few years later I stumbled across the first few chapters of the equally strange Uzumaki manga and was instantly hooked. Uzumaki is Japanese for "spiral", and the manga series deals with a small town cursed by a ubiquitous spiral shape, causing many of the townspeople - and seemingly the town itself - to go insane. Where the horror in Tomie involves a more straightforward single evil presence, Uzumaki is a more grandiose, Lovecraftian horror story. The entire manga can be read online here. (Remember to read manga from right to left.)
Several years ago, around Halloween, I came across the spectacularly creepy Enigma of Amigara Fault on a forum I frequented. If you're interested in reading any of Ito's work, this is definitely the place to start. It's relatively short (around 30 pages) and is a great example of some of Ito's recurring themes, such as inexplicable "natural" phenomena, compulsion, and body horror. Oh, the body horror.
This year while searching for the Amigara Fault manga, I found a fantastic resource containing a ton of Ito's work, including many published collections of short stories as well as the Tomie manga. The site also contains works by Nakayama Masaaki, who I'm completely unfamiliar with, but I plan to remedy that soon. Go check it out, and remember again to read from right to left.