45w, 5dTwilight's Library
A long long time ago, shortly after the three pony tribes had united and founded Equestria. A powerful draconequus named Discord, came down from the great mountains in the north where nopony has ever ventured.
Discord saw how the ponies lived happily in harmony, Discord was disgusted by the sight of the happy ponies. Using his powers of chaos and disharmony. He led Equestria into an era of pain and misery.
The pony goddess Queen Faust saw how her little ponies suffered, and decided she had to intervene. She took the strongest traits of the three races. The magic of the unicorns, the wings of the pegasi and the strength and resilience of the earthponies, and combined them, to create what we call alicorns.
Queen Faust made two alicorns, one pure white with a bright pink mane, she named this one Celestia, and the other a midnight blue with a light blue mane, she named Luna.
After losing countless battles against Discord, the alicorns sought out the ancient dragon prophet Luther, who lived to the south, far beond the Everfree forest.
Luther told the alicorn sisters of some powerful artefacts unknown to ponykind, they were called the elements of harmony. Long before the emergence of Queen Faust they had been taken by the death god Hades, who had hidden them in the endless depths of Tartarus.
The two sisters set out on a dangerous journey to the farthest reaches of the realm of the dead, here they faced countless dangers. After searching for months they finally found the elements.
Celestia and Luna then returned to Equestria, and using the elements of harmony, they defeated Discord and turned him into stone.
To make sure such harm would never come to her beloved ponies again, Queen Faust crowned her two daughters princesses of Equetria. Celestia was given responsibility of the sun, and Luna was given the control of the moon.
As they were granted control of our heavenly bodies, the both got their cutie marks, Celestia´s depicting the sun and Luna´s the moon.
Together the two princess led Equestria into a period of peace and prosperity.