Once upon a time, in the magical land of Equestria, there were two regal alicorn sisters who ruled together and created harmony for all the land. To do this, the eldest used her unicorn powers to raise the sun at dawn; the younger brought out the moon to begin the night. Thus, the two sisters maintained balance for their kingdom and their subjects, all the different types of ponies. But as time went on, the younger sister became resentful. The ponies relished and played in the day her elder sister brought forth but shunned and slept through her beautiful night. One fateful day, the younger unicorn refused to lower the moon to make way for the dawn. The elder sister tried to reason with her, but the bitterness in the young one's heart had transformed her into a wicked mare of darkness: Nightmare Moon. She vowed that she would shroud the land in eternal night. Reluctantly, the elder sister harnessed the most powerful magic known to ponydom: the Elements of Harmony. Using the magic of the Elements of Harmony, she defeated her younger sister, and banished her permanently in the moon. The elder sister took on responsibility for both...

it cuts to Twilight Sparkle reading a book telling the story.

Twilight Sparkle is a unicorn with a light purple coat, dark purple eyes, and a midnight purple mane and tail with light purple and pink stripes. and a cutie mark with a six-pointed pink sparkle surrounded by five small white sparkles.

“Sun and moon…”

Twilight Sparkle reads quietly. She looks at the sky.

“and harmony has been maintained in Equestria for generations since. Hmm... Elements of Harmony. Hmmm…. I know I’ve heard that before. But where?”

Twilight is seen galloping in the bridge and into an ivory tower with a library atop. She sees her little dragon assistant(and friend), Spike.

“There you are. Quick, find me that old copy of Predictions and Prophecies.”

Spike does so, and Twilight starts reading it…

“Mare, mare... aha! The Mare in the Moon, myth from olden pony times. A powerful pony who wanted to rule Equestria, defeated by the Elements of Harmony and imprisoned in the moon. Legend has it that on the longest day of the thousandth year, the stars will aid in her escape, and she will bring about nighttime eternal! Spike! Do you know what this means?”

before Spike could respond, Twilight quickly says,

“Take a note please, to the princess Celestia.”

Spike grabs an empty scroll and a quill pen, then Twilight starts saying what to write.

“My dearest teacher, my continuing studies of pony magic have led me to discover that we are on the precipice of disaster!”

Twilight says, pacing around.