Back when she young, Princess Luna was arrogant to her duties she never saw them as to her liking, ponies played after sister's Dawn but slept after her moonlight Dusk. Only seeing this as a offense to her princess hood. She became angry, fierce and dangerous, bringing herself to total anger she transformed into one figure made of pure dark evil.
She changed, figure and pony but more terrifying in that she would punish everypony for annoying her loving night sky, everypony became scared of Princess Luna but she was no longer called Luna, she has changed into Nightmare Moon. Celestia witnessed her sister's hate and anger change her so she went to help her sister through her unholy rage but Nightmare Moon did not listen.
So Celestia was forced to banish her sister from equestria to a place where she would fit in, the one place she loved to calm down and play, the Moon. Nightmare Moon was banished for one thousand years up in the sky, for one thousand years Nightmare Moon has become more aggressive, a thousand years later after her banishment she managed to escape from the cage and started consuming ponies in darkness.
Ponies become darker and more sinister to other ponies, harmony was never shown and they hated everything, Celestia saw this so she came back to challenge her evil sibling so she will calm Princess Luna's rage, after thirty minutes of leaving canterlot she'd seen what her sister has created, most of her guards were attacked by dark angry ponies who were just happy thirty minutes before.
She made her way towards her sister but she got captured just as she reached her by an ambush Nightmare Moon planned out, Nightmare Moon wanted revenge so she banished Celestia to the sun. Luckily a day before Nightmare Moon's escape, Celestia sent one of faithful students to poniville which that student knew about Nightmare Moon's escape plan so she planned to find six magic items known as the Elements of Harmony which Celestia first used to defeat Nightmare Moon.
On the same night of Nightmare Night's escape, Celestia's student named Twilight Sparkle, set herself on an adventure with 5 other ponies which she didn't need but they went to find the Elements of Harmony with her anyway. Nightmare Moon tried to stop them to proceed, two hours later Twilight and her companions found the Elements of Harmony in a abandon castle which Nightmare Moon waited for them, After a strong fight between Twilight and Nightmare Moon, Twilight's companions caught up with her and with them at her side Twilight felt the magic they were releasing and she used her to connect with them to the Elements of Harmony which then is shot down on Nightmare Moon.
By their magic of Harmony, Nightmare Moon was defeated and then was returned back to Princess Luna, Celestia returned from the sun after Nightmare Moon's defeat, Luna appear but was petrified, scared and sad about what she have done and what she became, she cries and apologises to Celestia for being so stupid and arrogant. Celestia forgives her sister and Twilight and her new found friends forgive Luna as well.
Returning to Poniville, a celebration of Harmony is set up, everypony who was possessed by darkness was released after Nightmare Moon's defeat, Luna was still depressed about what she did but everypony forgave her because they knew she wasn't herself. Luna has gained happiness but a small part of darkness was still concerning inside this Princess.
