In this behind-the-scenes explanation of the Mare in the Moon incident, follow Nightmare Moon and Princess Luna as they encounter strange, artificial beings from another world. A Portal 2 crossover. Contains two stories:

Of Cores and the Alicorn: After what seems like an eternity, Wheatley and Nightmare Moon cross paths and discuss their respective exile in space.

Guerrilla Corefare: Nightmare Moon is free, but the magical accident that released her did more than just send her back to Equestria. Now, from behind the scenes, Luna exploits Nightmare's newfound weakness in order to sabotage the plans for eternal night.

Note: As older stories (in fact, written before the debut of Season 2), these are fairly amateurish, relatively speaking, and contain speculative-at-the-time characterization for Luna. With the exception of a few minor edits, however, they have been submitted in their original forms for posterity's sake. This isn't Airstrip One, after all, and I don't work for the Party.