Princess Luna

Princess Luna, formerly Nightmare Moon, is a pegasus unicorn pony, the younger sister of Princess Celestia and the antagonist in the first two episodes of the series. She is transformed back to her former self by the Elements of Harmony and returns to her duty of raising the moon and stars over Equestria every night. As Luna, she appears to be quite younger than she was as Nightmare Moon.

Personality
As Nightmare Moon, she was vengeful, condescending, jealous, and angry. The legend that is narrated in the first episode tells that these negative feelings turned her into Nightmare Moon in the first place.

As Luna, she seems much more soft-spoken and gentle, and full of regret over her actions. Little is known about her personality in this form, but she seems sad and remorseful when she appears in Ponyville. When the ponies present her with a wreath of flowers, she seems very touched and happy.

Skills
Being one of the two pegasus unicorn ponies, Princess Luna possess magic far beyond normal unicorn ponies. She is able to control the moon and the night sky, bringing them out each night and then moving them away to make room for the dawn.

As Nightmare Moon, she is able to take on the form of objects and other characters, among them mist, thorns, and a trio of pegasus ponies who present themselves as The Shadowbolts.

One thousand years ago
The legend narrated in the first episode tells that one thousand years ago, the day and night cycle of Equestria was ruled by two sisters: Luna, the younger, who brought the night, and Celestia, who brought the day. However, Luna grew jealous of her sister: the ponies of Equestria all enjoyed and loved Celestia's daylight, but were all asleep at night. No one was awake to appreciate the beautiful night she had worked so hard to create. Eventually, this jealousy and bitterness drove Luna to become Nightmare Moon, and prevent Celestia from bringing forth the sun. She intented to keep Equestria in eternal night. Her older sister, with great sadness and regret, stopped Nightmare Moon by using the Elements of Harmony, and imprisoned her within the moon. Henceforth she became known as the Mare in the Moon and knowledge of her slowly drifted to the area of myths.

Escape
A thousand years passed. The studious Twilight Sparkle read about the legend which prophesized Nightmare Moon will return on the night of the Summer Sun celebration, held in honor of Princess Celestia and the longest day in Equestria. As foretold, Nightmare Moon escaped and soon appeared in place of Celestia at the festival.

Twilight was the only pony, with the exception of Celestia, who anticipated Nightmare Moon's return. but she did not warn anyone as Celestia encouraged her to use her time to make friends. Once again Nightmare Moon desired to bring forth eternal night upon Equestria. Twilight knew that only the Elements of Harmony could stop her, and set out on a quest to find them along with her five new friends: Applejack, Fluttershy, Rarity, Rainbow Dash, and Pinkie Pie. Overhearing this, the evil mare tried to stop them with different obstacles. After many hardships, the six ponies realized that their own virtues as friends were the true Elements of Harmony.

Using the six elements: honesty, kindness, generosity, loyalty, laughter, and magic, the ponies were able to transform Nightmare Moon back into Luna.

Forgiveness
Purged of her dark feelings, Nightmare Moon was transformed back into Princess Luna. Celestia offered her sister to put the past aside and start anew, with the two of them ruling Equestria together as equals. Filled with remorse and shame for her actions, a tearful Luna apologized, said she missed her and accepted. The two sisters were thus reunited and the balance of day and night was restored.

Luna was last seen the end of episode two, attending Pinkie Pie's party by her sister's side. She seemed sad about what she had done in attempting to shroud Equestria in eternal night, but is cheered up when the ponies of Ponyville welcome her.

Current Status
Though she has not appeared since the two-part pilot, throughout the series Luna is once again responsible for bringing the night. According to the show's creator, Lauren Faust, Luna is set to return sometime in season 2.

Appearances

 * S1E01: Friendship is Magic. part 1 as Nightmare Moon
 * S1E02: Friendship is Magic, part 2 as Nightmare Moon and Princess Luna

Trivia

 * Princess Luna, alongside Princess Celestia were both based on the Generation 1 Princess Ponies on the 1980's animated series.
 * Luna's name means "moon" in Latin. Interestingly, the Friendship is Magic book, which comes with one of the gift sets of the toys, references her as Selena, "selene" meaning "moon" in Greek.
 * When imprisoned in the moon, Luna appears as a dark area on the moon's surface. Similar shapes in astronomy are called lunar mare (Latin for "sea", pronounced "mahr-ey").
 * Nightmare Moon's name is also a subtle pun, being both "nightmare" and "night mare"
 * According to Lauren Faust, the show's creator, Luna was possessed by some outside force which made her into Nightmare Moon. However, this has yet to be acknowledged in the show.