A creature once roamed the lands of Equestria, an artificial beast made of darkness and fear. A shapeless creature so horrible, Luna herself shuddered as it invaded every night she made, and Celestia feared it more than any mortal threat.
This creature, this terrible abomination now thought of as stuff of legends, was dubbed The Nightmare. It slithered about the dark, devouring anything that its thousands of teeth could latch to, and feeding off of the fears of fillies and colts as they slept, becoming more terrifying with each new dream it consumed.
Eventually, Celestia finally confronted The Nightmare and sealed it in a cave, where it would never escape. Or so they thought. Today, though, Someone has broken that seal, and unleashed an ancient horror upon the world.
Now free, the formless aberration of darkness has but two things on its mind: Revenge, and food.

(Note, this story will update every Halloween, so no begging. It will not save you.)