Luna, one thousand years in the past, becomes the hideous Nightmare Moon and is banished by her sister. However, the reason why she changed so greatly, and the powerful forces at work behind the scenes have, up until this point, been lost to history.
Luna, one thousand years in the past, finds a strange star in the far reaches of Equestria, and is horrified at the implications.
Luna, one thousand years in the past, becomes jealous of Celestia's power.
A Nightmare is born.