Fallout Equestria: Fall of Heroes is a one shot FO:E sidestory written by Light Guardian.
Fall of Heroes conveys the final moments of the last hero of the Wasteland as he lies on the ground dying. The leader of the mysterious dark force responsible for the death of the last hero's team and the last hero's own fatal wounds exchanges words with the dying pony, revealing mysteries of a timeless conflict.
The time Fall of Heroes takes place is never specified. The author, however, has revealed that the story takes place a long time after the original Fallout Equestria by kkat. The entire story only consists of a few minutes in an unknown isolated area at night.
Fallout Equestria: Fall of Heroes can be read as a standalone one shot story, but it's actually part of bigger plot. Light Guardian revealed that Fallout Equestria: Fall of Heroes is a prequel/ teaser for a much larger FO:E side story. The mysteries in Fall of Heroes will be explained in the follow-up fic.
Light Guardian has said that whether the sequel will be written and published or not depends on the reaction to Fall of Heroes.