This is a "request".
One day, Princess Luna was flying around in the Bemuda Triangle. She was suddenly transported onto Earth, and, out of surprise, she flew all the way to where the Titanic was. The people on board the Titanic were very scared of her upon her arrival, but they eventually warmed up to her when it turned out that she was lots of fun. All the people on board were having so much fun that they didn't make any record of her at all, because she was so fun and all. On one of the parties, she managed to slip away from the rest of the people, and made her way up to the place where the Titanic was steered. She walked up to the wheel, and stared at it for a while. "Hello," she said to the wheel. It didn't respond, but she didn't expect it to either. "I've been waiting for you," she said, "I have been waiting for them to be all distracted, so I could be alone." Out of nowhere, lightning began zapping at her hooves, but she appeared to ignore it for the moment. The unmanned wheel turned slightly in alarm. She walked towards it, untill she could smell its polish. "I have been waiting for a chance... To spend time with you," she put emphasis on the last word, where she leaned forward and places both of her hooves on the wheel, and it instantly stopped panic-turning. By now, the lightning was moving up Luna's body, where her coat was starting to turn black, with her tail turning into a blanket of stars. The wheel instantly recognised it as belonging to Nightmare Moon, and desperately looked for a way to escape. Nightmare moon didn't notice its stable attempts to get away, as the lightning had fully enveloped her, leaving Luna's alter-ego in her previous place. "I have been waiting for so long for this! Finally, we can be together! After all this waiting!" She was shouting out loud now, no longer trying to keep quiet to avoid the ship's crew. The lightning started transfering from her hooves into the Titanic, and the whole ship turned into a briliant but deadly lightshow, which instantly fried everybody who was on board into ashes. The Titanic started turning black like the night, and large sparks were launching off it, into the sky, which made the Titanic feel amazing. Nightmare Moon had her eyes closed, and was commanding the lightning which was coursing through every ounce of the Titanic's being, giving it unimaginable pleasure. The Titanic was eventually completely covered in dark matter, making it black as the night, the same shade as Nightmare Moon herself. When she eventually opened her eyes, the lightning died down, and she was panting, out of breath, but with a big smile on her face. She let go of the wheel, and, exhausted, the wheel started "falling", endlessly turning into the left. Suddenly, it hit an iceberg, which sent Nightmare Moon falling to the ground from the impact. "Nooooo!" She shouted, but the Titanic started sinking! "My love! Why must it end this way! Out of everything that could happen, this is the worst! Possible! Thing! Why? Why must this happen to you? You were such a great ship!" Nightmare Moon had tears in her eyes at this point, but she was interrupted by the sudden gush of water that was filling the area. With one last look to the lonely wheel, which she just realised had been broken by the force of her weight, Nightmare Moon took to the skies, flying above the Titanic, to watch it sink. The lighting went over her again, and she was soon transformed into her old self, only this time she was taller, and her mane flowed like Princess Celestia's. She hovered there for a moment, before turning away and flying back to the Bemuda Triangle, where she went back to Equestria.
And that's why Luna looked different at her latest appearence.