Two halves of the same whole. Day and night, darkness and light. Everything has its opposite. Celestia and Daybreaker, Twilight and Midnight, Nightmare Moon and Luna. Everypony has their opposite. Imagine a character that could split ponies in half, could create evil versions of good ponies and good versions of evil ponies. Now imagine that character was an evil pony with a mysterious backstory who refused to be seen by anyone except the evil sides of those he had split. Paranoia about Celestia or something. He also happened to be behind the creation of Nightmare Moon. Now he and Nightmare have a plan that includes the bearers of the Elements of Harmony.