As all ponies know through History classes, a thousand years ago, Equestria was in turmoil. Attacks from both outside and inside threatened to fell Equestria.
As a counter measure, a troop of Elite Royal Guards for each Princess, Solar Priests for Celestia, and Night Keepers for Luna are chosen as their most trusted army, both for offensive and defensive purposes.
This story is about one single particular Night Keeper, one that had been raised by Princess Luna herself, the unicorn Morning Star.
A solid start, I love it!
The way you introduced the Princesses through description rather than name was an interesting move. I don't see that often anymore. Looking forward to the next chapter!