Nightmare Moon (Merging)

For other versions of Nightmare Moon, see here.

Nightmare Moon is Princess Luna's alternate personality. Initially just a collection of Luna's jealousy and rage, she developed her own mindset over time, eventually growing independent enough to use magic to break free from Luna's mind and take over Princess Celestia's. While she was initially considered a villain, she has since reformed. She initially shared control of her and Luna's body, however when this became cumbersome and - with the addition of Nightmare Moon falling in love - awkward, Twilight Sparkle and Crimson Nightmare helped to figure out a spell that could split the two into two separate, physical bodies.

Personality
Nightmare Moon has undergone a huge amount of change in relation to her personality. Initially, when Celestia was first forced to banish her to the moon, she was filled with so much jealousy and hatred that it consumed her, making her more or less those emotions incarnate. After many years of being suppressed by Luna, however, she came to develop her own personality.

As her own individual, Nightmare Moon has a far nastier temper than Luna, but still shares her alternate personality's caring nature - especially towards foals. She is also very serious most of the time - though has been shown to loosen up on occasion - and is often brisk, rude, and violently blunt. She still holds a great amount of disdain towards anyone who doesn't like the night, often using her powers to enter dreams to torment that pony, revealing her petty nature.

Despite her facade of boundless confidence, Nightmare Moon is secretly very insecure and vulnerable, mostly since she's never been anywhere but in Luna as a part of her for so many years. She also lacks a stable identity and sense of self, which contributes to her lack of confidence.

While she acts aloof about it, she greatly values Celestia's opinion of her - ironic all things considered. It hurts her more than she lets on that Celestia, her 'sister' of sorts, doesn't want anything to do with her initially. This is an example of how emotionally delicate Nightmare can be in relation to certain subjects.

Biography
Nightmare Moon was born out of Princess Luna's rage and jealousy over being excluded from her subjects' praises, while her sister, Princess Celestia, was showered in love and adoration. Luna's alternate personality only grew more malevolent as her sister failed to recognize what was going on, going so far as to make Nightmare think she was mocking her. After four years of internal seething, Luna relinquished control of her mind to Nightmare Moon, who then released all of her pent-up hatred on Equestria, casting the world into eternal night - Celestia, being weaker magically, was unable to force the sun to rise. Celestia confronted Nightmare, however she couldn't reason with her, and the two fought. Initially Nightmare was winning, but then Celestia used the Elements of Harmony to banish her to the moon for a thousand years. Once banished to the moon, Luna regained control of her mind, leaving Nightmare to brood in the back of her mind. Over several hundred years Nightmare began developing her own unique mental state, and began lusting after revenge against Celestia and sole leadership of Equestria. Eventually she emerged from her corner of Luna's mind, reclaiming their body for herself. She learned over the years from studying the stars that when certain stars came into a certain arrangement, forming a unique constellation, her powers were enhanced. She formed a plan to use a particularly powerful constellation that only appeared once every few thousand moons to return to Equestria. A few hundred years later, on the eve of the thousand year anniversary of her banishment, Nightmare set her plan in motion. Using the constellation she managed to escape from her prison, then she ambushed Celestia and kidnapped her, gloating over her inevitable victory and attempting to force the location of the Elements of Harmony that posed such a danger to her out of her sister. However Celestia had long since lost track of them, and Nightmare was forced to rely on Twilight and her friends to find them in her place. Once the Mane 6 had located the Elements, Nightmare thought she would be able to easily claim them for herself. What she didn't take into account, however, was the fact that Twilight and her friends were the Elements' destined wielders, most notably Twilight herself, being the embodiment of magic. The spell Twilight and her friends cast on Nightmare allowed Luna to emerge from the depths of her own mind, where she fought with Nightmare to return to the mare she was before her negative emotions took over. In the end Luna won, and she sealed Nightmare Moon inside her for good. Or so she thought. During the battle against Malevolence, Nightmare Moon broke free and used her magic to transfer herself to Celestia's mind. The sun alicorn, inexperienced in fighting off Nightmare's attacks, succumbed to her control. Ultimately Luna wound up fighting her own inner demon who had taken control of her sister. Sometime during the battle Luna convinced Nightmare to surrender control of Celestia and return to Luna's body. Nightmare and Luna's relationship began improving after this incident, and eventually they began sharing control of her body. This ultimately proved problematic, however, as they both had too little time to accomplish anything - as they were splitting control, and thus the day. An even bigger problem arose after Nightmare fell in love with a stallion, even while Luna was married to Litch. This prompted them to seek out a solution, one which Twilight and Crimson delivered in the form of a high-level magic that gave Nightmare a corporal form.

Abilities
===Mental === Nightmare Moon has the same level of intelligence as Luna, making her quite a bit above average intellect. However her haughty, over-confident, and sporting nature tend to be her downfall when it comes to using those smarts to accomplish her goals.

===Magical === Nightmare Moon has the same level of magical power as Luna, however unlike Luna she has no qualms with releasing it all without restraint. This makes her one of the most powerful ponies alive, though not on the level of figures such as Astral, Star Swirl the Bearded, and Crimson Nightmare.

===Physical === Nightmare Moon has the same level of physical ability as Luna. Being an alicorn, Nightmare has the Earth Ponies' natural strength. Still, as far as female alicorns go she's not all that strong, lacks endurance, and has little in the way of flexibility. Some ponies suggest this is because Luna uses all of her free time playing games in her room.