A Metro 2033 crossover.
Seventeen years ago, Princess Twilight Sparkle took her last stand at the Crystal Empire against the Griffon invaders when the impossible happened: the Crystal Heart was destroyed.

The result was a cataclysmic burst of magic which made the surface uninhabitable, driving what remained of the pony race underground into the new Equestrian Metro system.

After years of new factions and bloodshed, Ponyville station has come under threat from a strange new species of mutated ponies: the Dark Ones.

Following her return, Apple Bloom has tasked Scootaloo's adopted daughter with telling the council of Canterlot of the dangers coming from the Everfree forest, before Ponyville and the rest of Ponykind falls prey to this new threat.

But the journey from Canterlot to Ponyville is not a short or safe one. Years of inhabitance in the Metro has changed ponies, so her journey will be anything but easy.

No longer is Equestria the utopia it once was.