Zuli Archer is a name known only to the most elite of the unicorn students of Celestia. He once was one of the few things that stood in the way of Nightmare Moon herself during her initial attack. He was only defeated due to an incomplete spell of his own design.
Freed from Nightmare Moon 1000 years later much as Luna was, Zuli returns to the world a changed unicorn. With this experience behind him, Zuli sets forth to train many a new unicorn or Pegasus in his ways so as to leave his mark for another reason.
A fifth of his kind, one that has witnessed a thousand years or more, shall soon come again. A pony that even a somepony as ancient as Zuli does not remember.
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Author's Note:
If you want a more accurate and detailed backstory for Zuli, my friend is currently writing it. Once its uplaoded, I'll put a link for it here.