Long ago, 300 years before the young Princesses of Equestria existed, there was an Alicorn clan of Knights in service to Queen Cosmos. The queen, young and selfish at the time, wished for their leader to become her King. The Leader, Meteor Knight, who already had a family of his own, rejected Cosmos' offer. She, enraged by the refusal, immediately banished the entire clan without a second thought, and ordered all of their bloodlines to be terminated for all of existence, and cut their name to be left as Night, the shadow of the sun covering their leave as though an ominous cloud. Three hundred years after, Cosmos fell ill after giving birth to Princess Luna, and Celestia - a young filly at the time herself - was taken on a Knight Hunt with her mother's 'Day Ponies', the elite group sent to kill the clan. Horrified at what she'd seen, Celestia ordered the group to disband and for the Clan to never be hunted again. She forgot, however, to send a messenger to the Night Clan. See here how one mistake of a lifetime can disrupt a family's uneasiness and unite Night with Day.