It had been 1,500 years ago since Princess Celestia and Princess Luna attempted to defeat the queen of Unicornia but before they could, she cast a spell to cause it to disappear. Unicornia is back now and so is its queen. Princess Celestia has tasked Twilight and her friends with reuniting her with its leader but this will lead to so much more than a mere reunion.

Cover art source: https://sourcemlp.fandom.com/wiki/Unicorns?file=Unicorn_banner_vector.jpg