These are short stories to the fic Princess Celestia: The Changeling Queen. They document the fun stuff, the struggles and the touching moments of a changeling queen in her most challenging impersonation ever.
Under threat from a possible war between Princess Celestia of Equestria and Oda Nobunaga of Japan, William the First of the Dutch invites the bickering world leaders to a peace talk. Stupidity ensues.
Monster finds problems fitting in and getting used to her new world in Ponyville. To help adjust, she reaches out to Princess Luna who has many of the same problems now that she is recovering from being Nightmare Moon.
Princess Celestia and Pinkie Pie have a Freaky Friday Flip and Twilight and her friends must restore them before a thousand year old acquaintance of Luna turns her back into Nightmare Moon and bring back eternal night.
After Celestia humiliates her at a dinner, Luna storms off and ends up snooping around in her sister's room. There, she stumbles upon the journal Celestia kept immediately after the banishment of Nightmare Moon.
Twenty years before the Elements of harmony, before Nightmare Moon's Return, Celestia started a whole new slew of problems and headaches for herself and the ones around her. Thankfully Cadence doesn't mind foalsitting.
In the final years of her imprisonment, Nightmare Moon was still plotting, but she wasn't returning alone. Behold, the Heir of the Moon! MLP/HP crossover