The Sun and Moon are gone.

Equestria was plunged into a deep winter, a cold, heartless wasteland. Ponies had to do something. So they retreated underground, establishing a mining colony that stretched across the cities of Equestria. They started with isolated sections and slowly built across, creating many tunnels that stretch between the cities. Everything was heated through magical generation built with a mix of pony ingenuity, technology, and magic.

That failed. Everything is cold. It's warmer than the surface, but ponies die of hypothermia daily. Without heat, food stopped growing, and water froze, and more wasn't able to be processed from the snow above. Ponies hurt each other, forces into shattered remnants of what was once a happy society, living in peace and harmony.