The Chronicles of Foxton
Leth "Encephilitus Lethargicus"
The creature known as Encephilitus Lethargicus is a demon lord who was summoned to this plane through some unknown means. It possessed an unwitting human shell and called itself “Leth,” then recruited a pair of unscrupulous alchemists (Curry and Burglesmith) to distribute a dangerous drug of its own creation — Boo Powder.
The demon was driven from this world after its schemes unraveled and its human dupes were killed in a fire.
Encephilitus Lethargicus is also known as the Oinerodemon, the Sandman, the Boogeyman, the Nightmare, and several other names. It is an ancient creature, having been first encountered in our world thousands of years ago. The name that most sages refer to it as is from the Elvish, and means literally “Sleeping horse-head.” It frequently appears, in dreams and in the waking world, as a huge black horse.