This is how Nightmare Moon tricks Twilight goes in
[Nightmare Moon appears]
Nightmare Moon: Hello.
Twilight Sparkle: Nightmare Moon!
[She prepares her magic]
Nightmare Moon: Wait!
Twilight Sparkle: Why?
Nightmare Moon: Because I am your friend. Unlike those so called-friends of yours and your brother. They wouldn't believe you about that fake Cadance being evil, would they?
[Twilight looks on]
Twilight Sparkle: [sighs] No, they wouldn't.
[She begins to leave]
Nightmare Moon: They might not, but I believe you.
[Twilight stops and looks back]
Twilight Sparkle: You do?
Nightmare Moon: Yes.
Twilight Sparkle: The fake Cadance. Do you know who she really is?
Nightmare Moon: She is the Queen of all Changelings, Queen Chrysalis.
Twilight Sparkle: That would explain her odd behavior.
Nightmare Moon: That's why I got a very important job for you.
Twilight Sparkle: What kind of important job?
Nightmare Moon: Come with me if you want to find out.
Twilight Sparkle: Alright.