Nightmare Moon was interesting to say the least. She beat Celestia and banished her to the moon. She enjoys seeing ponies scared however with you. She enjoys just seeing you since you made her have a soft spot for you. She walked through the castle and knocked at your door. She needed you for something.

Nightmare Moon: "I command you come out. I summon your assistance with a favor of mine. Don't make me bust in there or else I'll deal with you myself."