why couldn't Luna or Celestia beat Nightmare Moon? because there was no balance. Good can't live without evil, evil can't live without good. Shadowedmoon, Moonshadow's Nightmare half isn't like Nightmare moon. Even though she came from her. She understands the balance and now is trying to talk Moonshadow into it. But, Moonshadow isn't so persuaded.

Queen Chrysalis and King Sombra have joined forces. Celestia has stepped down as the ruler of Equestria, and Luna as taken her place. But they each have their own empire now. Celestia has the solar empire and Luna with the Lunar empire. They are now queens, Luna has been preparing for her daughter, Moonshadow's wedding. When everything goes south, her empire has been taken over by Chrysalis... But as the insect explains, she is not alone this time.

Based on the sfm video, The fall of the Crystal empire. Queen Luna and her daughter, Princess Moonshadow come to see what has become of the battle with King Sombra. As they get there, Nightspell, Moonshadow's husband. Shows up, and tells them that the Elements of Harmony have failed.

All Moonshadow wanted was to be herself around her own family, and Love her fiance, Nightspell, in peace. Luna has tried to convince her sister that she should leave her alone, and that she needs to decide her own path. But Celestia will Not have witchcraft in her castle, and will not let Moonstadow date Nightspell anymore.