In almost every universe, there are a group of individuals known as Reapers. Elite Warriors, heroes and the bane of civilisation are a few of the things people have called them. But, perhaps their redeeming feature is their honour and comradery. If one falls, all of his brethren shall rise to avenge him. So how does a universe lose all but one? On Earth, the planet is gradually being consumed by Equestria, humans forced to convert, or die. Desperate, the last Reaper crossed the dimensions to a different Equestria, seeking help from the only Equine Reaper Corps in existence.
Celestia, seeing her universe's Reapers number only six, summons four humans from the Twelth Reaper Corps; renowned for their daring and sense of humour; to assist. But what horrors truly lay at their destination, none could have guessed...

(TCB Halloween event fic. Spoilers in-story for anyone who really thinks about it.)