A large part of Nightmare Moon still remains in Luna's blood, as she reforms her dreams into nightmares. Luna becomes threatened with the consequences against her sister if she does not respond to and obey Nightmare Moons orders. If Luna as so tries to free herself from slavery, Nightmare Moon has sworn to return to her body, it will be definite that Luna shall be banished for eternity.
May Celestia have mercy on her soul, forgive her.