Basically, this is an AU where Equestria became a Wasteland after a bomb goes off, killing or changing anyone who was exposed. Heart Mouse, Small Brooke, and Bittersweet survive the bomb by taking shelter underground but their family and friends weren’t so lucky, causing them to loose their minds over time. Heart Mouse became Death Mouse due to grief, and Small Brooke became mentally unstable, causing her eyes to turn black and have black tears. Bittersweet is already cursed so she has a corrupted form but she’s trying her best to stay sane, but eventually that will be lost.
 
 
This is similar to a Fallout Equestria but worse.