Falling Shadows by Drako Moon & Stardust Night:
https://www.fimfiction.net/story/350442/fallout-equestria-falling-shadows

The Authors of Falling Shadows also have a Patreon. If you enjoy their story, feel free to check out the Patreon below:
https://www.patreon.com/FoEFallingShadows?alert=2