A life underground is all Celestia and Luna have ever known.

The world above is twisted and deformed. Day and Night take turns at random. A clear summer day can turn into a blizzard in the blink of an eye. The ponies that inhabit this world are corralled and controlled by Discord, a maniacal god of chaos. But some ponies resist by living underground where the world is stabler, where time can be measured properly and a semblance of normal life can be lived. Celestia and Luna believe that they are normal ponies, like all the others hidden below. But in truth, they have the power to change the world forever, they just need to beware, and ensure that what they are, does not change who they are.

Cover art is "Final Stand" by greennpc
https://www.deviantart.com/greennpc/art/Final-Stand-534330857

Note: This is an alternate, anthro universe re-imagining of the MLP: FIM Backstory/Lore we've gotten on the show over the years. As opposed to me filling in the blanks I'm taking all the disparate pieces and rearranging them for fun. A different twist of familiar characters and stories if you will.
(Psssst mate, that's just all of fan fiction)
(Oh crap, you're right, uh..........*JAZZ HANDS*)
Also I'm marking this story as Mature even though it might not seem like it at first.