Flutter Night was born with a demon, known as Seraphic Nightmare. Unlike most chaotic sides which appears rarely, Flutter Night's chaotic side appears every moon. But her chaotic side turns her into a merciless killer, known as Seraphic Night, which can only be defeated and calmed down by Flutter Night, herself. When she is Seraphic Night, her true self doesn't know, but memories start to appear in her mind. Seraphic Nightmare, afraid that once Flutter Night finds out about her chaotic side, there will be nopony to save her. To save Flutter Night before it happens, Princess El asks four ponies to help.
(Seraphic Night doesn't really look like Nightmare Moon, but she resembles it, I guess)