Yet another bit from i09.com:

http://io9.com/two-sentence-horror-stories-are-actually-pretty-chillin-923728355

I loved the one about the kid and the “monster” under the bed.

Some of the comment writers to that article have referenced Fredric Brown’s classic short story Knock, with its short-short story opening, as a classic example of a very short horror story.  Knock opens with this:

The last man on Earth sat alone in a room. There was a knock on the door…

Interesting to note, at least according to Wikipedia that this famous opening line was itself apparently cribbed from the following, originally written by Thomas Bailey Aldrich:

Imagine all human beings swept off the face of the earth, excepting one man. Imagine this man in some vast city, New York or London. Imagine him on the third or fourth day of his solitude sitting in a house and hearing a ring at the door-bell! (Ponkapog Papers, 1904)

Chilling stuff.