Nightmare Moon

Appearance.

Her eyes are a moderate cyan, and her mane is a brilliant phthalo blue with grayish persian blue outline. Her coat is a very dark cornflower blue and her eyebrows are a moderate blue violet and her magic aura is a brilliant cyan with light violet sparks.

Personality.

Much like many archetypal, over-the-top villains, Nightmare Moon is overly confident of herself and dramatically declares that the protagonist is a fool (foal) for thinking she can defeat her, and that "the night will last forever." She is portrayed as vicious and maniacal, shown when she attacks Princess Celestia and cackles evilly in Princess Twilight Sparkle - Part 2.

Bitterness.

The narration at the beginning of Friendship is Magic, part 1 speaks of Luna's bitterness which transformed her. When she first appears, she intimidates the ponies with her evil laugh and pointed questions. She snidely refers to the ponies as her "beloved subjects," but appears genuinely impressed when Twilight Sparkle recognizes her as Nightmare Moon.

Manipulation.

Nightmare Moon uses her magic to set obstacles in Twilight and her friends' path to the Elements of Harmony indirectly, making creatures unwittingly assist her. These include the manticore, whose paw she embeds with a thorn so he'd attack the ponies, or Steven Magnet, whose mustache she cuts off so he'd make the river impassible. She further indirectly attacks the ponies by causing a cliff side to collapse underneath the ponies' hooves and making trees look like monsters. As the Shadowbolts, she tries to persuade Rainbow Dash to abandon her friends so they won't be able to cross a fallen bridge and reach the Elements of Harmony. Despite her indirect approach, when she is attacked by the royal guards, she quickly repels them with lightning, then turns into mist and floats away. She also confronts Twilight Sparkle directly when the two charge at each other, right before shattering the Elements of Harmony.

Depiction in the series. (Season one)

Princess Luna is introduced in the prologue of the first episode as the younger of two regal Alicorn sisters who "ruled together and created harmony for all the land." Luna was in charge of bringing the moon out at night, while her sister, Princess Celestia, raised the sun at dawn. The narration explains that their pony subjects played in the day but "shunned" and slept through the night, causing Luna to grow bitter. After refusing to lower the moon one day, she transformed into Nightmare Moon, a "wicked mare of darkness," compelling her sister to banish her to the moon by wielding the power of the Elements of Harmony. Following the prologue, Twilight Sparkle looks up a prophecy about Nightmare Moon that refers to her as the "Mare in the Moon" and states that, "on the longest day of the thousandth year, the stars will aid in her escape, and she will bring about nighttime eternal." Twilight observes that the upcoming Summer Sun Celebration will mark the one thousandth year, and as predicted, Nightmare Moon appears at the Celebration in Ponyville and declares that the night will last forever. At the end of the second episode, Twilight and her new friends confront Nightmare Moon and use the Elements of Harmony to transform her back into Princess Luna, who then remorsefully accepts Princess Celestia's offer to rule by her side again. The show's developer, Lauren Faust, explained in a comment on her DeviantArt page that Luna was "no longer present" after transforming into Nightmare Moon, that her goodness could not have been "drawn out" while she was banished for the thousand years, and that Princess Celestia "did what she could" to orchestrate the events that eventually broke jealousy's spell over her sister. Faust added that this explanation is "as far as [she] ever got with it," and that the explanation might unfold differently on the show.

Depiction in the series. (Season five)

In The Cutie Re-Mark - Part 2, due to Starlight Glimmer's alteration of the past, she inadvertently creates a future where Nightmare Moon deposes Princess Celestia, banishes her to the moon, and brings about eternal night. Catching word of Twilight Sparkle's method of time travel used to stop Starlight, Nightmare Moon expresses interest in using this magic to prevent the Elements of Harmony from being found, ensuring her eternal rule. However, Twilight gets away and rewrites the timeline before Nightmare Moon can act upon it. In a second alternate timeline, Luna and her sister fail to defeat Discord and are tormented by him as he rules Equestria.

Magic. (Transformation.)

On multiple occasions, Nightmare Moon turns her entire form into mist. She appears in this manner in Ponyville's pavilion where the Summer Sun Celebration is being held, and she follows the Mane Six on their journey for the Elements this way. While in this form, Nightmare Moon manipulates creatures and environments; she transforms into a thorn to hurt a manticore, prompting it to attack the Mane Six, and she transforms into the Shadowbolts at one point.

Magic. (Weather Manipulation.)

Nightmare Moon uses lightning to repel the royal guards in Friendship is Magic, part 2 and thickens the mist over the fallen bridge later in the episode to prevent Rainbow Dash from hearing Twilight Sparkle calling out to her.