As far as Equestrian History spans, The only Alicorns in existence were in the Royal Family. But one day, this exception changes not only Celestia's and Luna's lives, but also The Mane Six's as well once a little colt, hood hung over his face shows his face in Ponyville, as well as Twilight's life. Sent by Celestia, the young Alicorn is instructed to learn from the Mane Six, specifically Twilight; but once she gets accustomed to him, she begins to question his true purpose there.

Gdocs version here: https://docs.google.com/document/d/1dUeoLwaWdXJVeyL0XWWqeidXWYiB9CoUUnJv_x8EJQI/edit?pli=1