MLP Villain Timeline- Nightmare Moon
1. Foreshadowing and Seeds Planted
In the mini-series comic, Legends of Magic #1, a teenage Luna magically opens up a portal to the alternate Pony of Shadow’s dimension where she was kidnapped and almost turned into Nightmare Moon earlier. She opened the doorway because she wanted to practice a harder spell so that she could stop Celestia from taunting her because of Luna being less skilled at magic than Celestia. Once she opens the portal and hears voices telling her that she’s destined for greatness, wanting revenge on her sister, and having dark magic inside of her. But she vows that she would never turn on her Celestia.
So Nightmare Moon didn’t make her debut in the main timeline, but the seeds were planted from Celestia’s taunting to Luna opening the dark dimension, so indirectly Celestia is responsible for Nightmare Moon.
Celestia and Luna get kidnapped again by the minions of the alternate Pony of Shadow and taken to the same dark dimension and the alternate Pony of Shadows wanted to corrupt them with shadow magic turning them into Daybreaker and Nightmare Moon, but the Pillars managed to rescue them in time and was saved.
2. 1,000 Years of Inner Resentment
After the Pillars from the main series banished themselves and Pony of Shadow, Celestia and Luna were left with Equestria as their kingdom. Over that time, the sisters ruled Equestria and stopped many threats and were celebrated. But internally, somewhere along the line she started to grow jealous and resentful over the attention and praise that Celestia received and how ponies played in the day and slept during the night. This led to the downfall of Luna and the rise of Nightmare Moon. But some of the impacts of Luna’s life before her resentment towards Celestia were:
Defeating Tirek and banishing him to Tartarus and befriending Scorpan alongside Celestia
Despite (losing her memory) helped Celestia, Novo, Aspen, Discord, and the King of Abyssinia banish Cosmos to the stars
Celestia and Luna use the Elements of Harmony to defeat Discord and turn him to stone
Celestia and Luna gather the Elements of Harmony from the Tree of Harmony and stored them at the former Castle of the Two Sisters
Celestia and Luna got word from Radiant Hope, a former student of their’s and was informed that Princess Amore was turned into dark crystal and shattered by Sombra, and the two went to the Crystal Empire and used their own magic and turned him into shadow
Although, Sombra was defeated the sisters suffered a huge loss with the Crystal Empire being lost for 1,000 years and their friend Princess Amore being presumably killed
3. The Rise of Nightmare Moon
Some time after fall of the Crystal Empire, Luna started to be distant with Celestia as ponies played in the day her sister brought forth, but shunned her night and slept through it. One day in the Castle of the Two Sisters Luna bitterly went on about being in Celestia’s shadow and having her night overlooked by Celestia’s day and vowed that she would be the only princess in Equestria. With that, she rose the moon which eclipsed the sun, and that darkness mixed with Luna’s bitterness and jealously, turned her into a wicked mare of darkness proclaiming that she will bring about night time eternal and destroy Celestia, this was the birth and rise of Nightmare Moon.
Note: I want to headcanon about Luna turning into Nightmare Moon by tying in the show and comics. So I headcanon that Luna had over 1,000 years of bitterness and jealousy over Celestia and it was strong enough to attract the Nightmare Forces which are the dark counterparts to the Nyx who are the Guardians of Dreams. Both live on the moon but while the Nyx live on the light side of the moon, the Nightmare Forces live on the dark side of the moon. The Nightmare Forces surrounded Luna and enticed her into giving her their dark energy alongside her resentment and jealousy consuming her with that mixture she turns into Nightmare Moon.
4. Nightmare Moon’s Banishment
No longer Luna, Nightmare Moon began to ruthlessly destroy the Castle of the Two Sisters and aiming her sights on Celestia to destroy her, and claim Equestria as her own Lunar kingdom. She battled against Celestia and managed to get a good hit on her but Celestia used the Elements of Harmony to banish Nightmare to the moon for 1,000 years before she could really do any damage, however it was the darkest moment of Celestia’s life…but it didn’t stop Nightmare Moon.
In the mini-series Fiendship is Magic Issue 4, Nightmare Moon after getting banished to the moon, ends up finding a castle on the moon where it’s inhabited by the Nyx, creatures who are responsible for creating the dreams of ponies. She manipulates them into showing her how the dream and nightmare process works in order to try and get another jab at Celestia by messing with her dreams, but Celestia’s mind is too powerful so Nightmare goes after the ponies of Equestria instilling fear into them about Celestia and having Nightmare being regarded as a hero in the real world but Celestia puts a stop to it, but leaves her mind vulnerable. Nightmare then betrays the Nyx and corrupts them into serving her and giving her the power to get into Celestia’s dreams…literally. Nightmare ends up getting into Celestia’s dreams and tries to use Celestia’s guilt over losing Luna to the darkness but it only strengthens Celestia’s resolve and she defeats Nightmare and breaks the connection to her dreams from the Nyx so that Nightmare can’t enter her mind, but it doesn’t stop Nightmare from going into other’s ponies dreams and causing fear and nightmares.
Nightmare Moon at this point is able to learn the ability to see ponies dreams and even literally be inside their dreams and interact with things
Nightmare is also aware of the dream process and how it works due to the Nyx
By the last line of the comic it’s implied that she tormented ponies in their nightmares for 1,000 years
6. The Legend of Mare in the Moon/Nightmare Night
Over the 1,000 years of banishment of Nightmare Moon, Celestia continued to watch over Equestria peacefully over the millennia and legends and stories were created one of the wider known legends was the Legend of the Mare in the Moon. Now for Nightmare Moon to have a legend and even a holiday centered around her, ponies must’ve heard about her from somewhere and this comes in form of her dream-walking abilities. If ponies weren’t going to love her, she would make them fear her. The tale of The Mare in the Moon as well as the festival Nightmare Night and The Summer Sun celebration were holidays that were celebrated for 1,000 years to the point where Nightmare Moon became a tale for fillies and colts.
7. Nightmare Moon’s Return and Defeat
1,000 years later at the Summer Sun Celebration on the longest day of the thousandth year, Nightmare Moon was prophesied to return to Equestria and be free from her banishment on the moon and had another chance to bring about eternal night. She escaped via the stars and went after Celestia before crashing The Summer Sun Celebration in Ponyville. She then announced to the Ponyville citizens that night will last forever, but one pony wasn’t having it and her name was Twilight Sparkle. Nightmare followed Twilight to her temporary home and finds out about how the Elements are located in the old castle of the two sisters. Nightmare does whatever she can to stop the six ponies from trying to reach the castle such as:
Turning into a thorn to poke a manticore, causing it to attack the girls
Ripped off a sea serpent’s mustache
Scared the six with evil trees
Trying to trick Rainbow Dash with shape shifting
Destroying the Elements in their stone forms
Unfortunately for Nightmare Moon, the ponies that she tried to stop ended up being the spirits to the Elements despite the physical gems being destroyed. The six ponies came together and reconstructed the Elements of Harmony and they used it’s power to blast Nightmare Moon destroying her and purifying Luna back to normal. This was the last of Nightmare Moon and the return of Princess Luna from over 1,000 years.