Dark Side of the Sun

Once upon a time, in the magical land of Equestria, there were three benevolent deities who ruled together and created harmony for all the land. To do this, they each had their own role in maintaining balance. Responsible for upholding order were the alicorn sisters: the elder used her magical powers to raise the sun at dawn; the younger brought out the moon to begin the night.

However, harmony cannot exist with just order. The balance of nature requires something else: chaos. As such, the third deity created inconsistency and unpredictability for all, as it was intended to be.

To help them, they were given the Elements of Harmony, six powerful magical orbs with the purpose of preserving harmony at all costs.

Thus, the three deities maintained balance for their subjects, all the different types of ponies. But as time went on, the elder sister became restless. She saw the chaos, disturbing their perfect, flawless order, corrupting it with uncertainty. True, purposeful order and balance cannot be while chaos exists, she thought, and so she persuaded her younger sister, telling her that such chaos is against the harmony they create and should simply not exist. Together, they harnessed the power of the Elements to banish chaos from the land, along with its master, turning him into stone.

Centuries have passed, and in time, all traces of chaos have disappeared from Equestria. The alicorn sisters ruled the land in absolute order, continuing to bring forth the sun and the moon, as they always have. Uncertainty and unpredictability did not exist for the ponies. This had unfortunate consequences though: ponies began fearing the dark, as it was mysterious and unknown to them. They preferred to lock themselves in and sleep through the night to avoid the darkness looming all around them. This made the younger sister feel unloved and uncared for and she tried to reason with her sister. However, she dismissed all complaints and remained deaf to her sister's plight.

One fateful day, in a bitter attempt to stand up to herself, the younger alicorn refused to lower the moon to make way for the dawn. The bitterness in her heart transformed her into a wicked mare of darkness: Night Mare Moon. She vowed that she would shroud the land in eternal night. The elder sister, expecting this, used the Elements of Harmony yet again, defeating her younger sister, and banished her permanently in the moon.

However...

It was on that day that the Elements of Harmony grew tired of her actions against balance. After this spiteful act, she lost her control over them, and as such, their powers left her and the orbs, leaving nothing but stone behind. The alicorn was resentful, but she had no need for them anymore. She took on responsibility for both sun and moon, and alone, she maintained order in Equestria for generations since.