This is how Twilight becomes Nightmare Moon's apprentice goes in My Little Pony Transformers: Revenge of the Sith.
[We then see Twilight walking through the halls of the Senate]
[Suddenly, she comes across Nightmare Moon]
Twilight Sparkle: Nightmare Moon! What are you doing here?!
Nightmare Moon: Easy, Young Sparkle. I've only come to talk.
Twilight Sparkle: Why should I listen to anything you say?!
Nightmare Moon: Don't you want to know why the Council doesn't trust you?
Twilight Sparkle: They trust me. They don't trust you.
Nightmare Moon: Be careful of the Jedi, Twilight. Only through me can you achieve a power greater than any Jedi. Learn to use the Dark Side of the Force and you will be able to save your husband from certain death.
Twilight Sparkle: What did you say?
Nightmare Moon: Use my knowledge, I beg you.
Twilight Sparkle: [gets out her Keyblade] You're the one who started this war.
Nightmare Moon: I know what's been troubling you. Listen to me. Don't continue to a be a pawn of the Jedi Council. Ever since I've found out about you, you have been seeking a life of great significance, far more than any Jedi.
Twilight Sparkle: I have to admit. My trust in them has been shaken.
Nightmare Moon: Then join me.
Twilight Sparkle: Why?
Nightmare Moon: Because you need me.
Twilight Sparkle: Why should I believe you?
Nightmare Moon: Because I can help you save Flash Sentry from certain death. [pauses for a moment as Twilight holds her Keyblade at the back of her neck] Are you going to kill me?
Twilight Sparkle: I would very much like to.
Nightmare Moon: I know you would. I can feel your anger. It gives you focus, makes you stronger.
[Twilight puts away her Keyblade]
Twilight Sparkle: I'm going to report you to the Council.
Nightmare Moon: You will try. Learn to know the Dark Side and you'll be able to save your husband. You have much wisdom, Twilight. Learn the power of the Dark Side, Twilight. The power to save Flash.
[Twilight walks away]
[We then see her heading back to her transport]
[We then see Twilight walking up to Ventus, who is leading Sylveon, Brian, and Anakin to go check on the Senate's motives]
Twilight Sparkle: Master Ventus.
Ventus: Sparkle, we just received word Ryan has destroyed General Grievous. We're heading over to the Senate to inform them. Their reaction should give us a clue to their motives.
Twilight Sparkle: Master Ventus, I have just learned of a terrible truth. Nightmare Moon is behind everything.
Ventas: Nightmare Moon? Are you sure?
Twilight Sparkle: I encountered her on my night walk.
Ventus: Then our greatest fears have been realized. We must move quickly if the Jedi Order is to survive.
Twilight Sparkle: I should go with you.
Ventus: No. You're staying here.
Twilight Sparkle: But I've told you everything I've learned.
Ventus: If what you say is true, then you will have earned my respect. But for now, wait for me at the Temple.
Twilight Sparkle: Yes master.
[Twilight then hears Nightmare Moon's voice]
Nightmare Moon: (voice) You do know, if they kill me, you'll never be able to save your husband.
[Twilight gets up and leaves the room]
Ventus: You are under arrest, my lady.
[Nightmare Moon backs to the edge of the window]
Nightmare Moon: Please, don't kill me.
Ventus: The oppression of the Sith will never return. You have lost.
Nightmare Moon: No. No. No. You will die!
[Nightmare Moon fires lightning at Ventus, who blocks it with his Keyblade]
Nightmare Moon: [to Twilight] He's a traitor!
Ventus: She is the traitor! [grunts]
[Twilight looks from one to the other]
Nightmare Moon: [to Twilight] I have the power to save the one you love. You must choose.
[Twilight looks at Ventus]
Ventus: Don't listen to her, Twilight! [groans]
Nightmare Moon: Don't let him kill me! [releases the lightning] I can't hold it much longer. I–I–I–I can't. I–I–I'm weak. I'm–I'm too weak.
[Twilight and Nightmare Moon look at each other]
Nightmare Moon: Twilight, Twilight. Help me. Help me. I–I–I can't hold on any longer.
Ventus: I am going to end this, once and for all.
Twilight Sparkle: You can't. She must stand trial.
Ventus: She has control of the Senate and the Kingdom Hearts Republic. She's too dangerous to be left alive.
Nightmare Moon: I'm too weak. Oh, don't kill me. Please.
Twilight Sparkle: It's not the Jedi way. She must live.
[Ventus raises his Keyblade]
Nightmare Moon: Please don't.
Twilight Sparkle: I need her.
[Ventus charges his Keyblade with his powers]
Nightmare Moon: Please don't!
Twilight Sparkle: No!
[Twilight gets out her Keyblade and cuts off Ventus' arm]
[Nightmare Moon fires lightning at Ventus]
Nightmare Moon: Power! [fires more lightning at him]
[Twilight puts away her Keyblade, and falls to her knees]
Nightmare Moon: Unlimited power! [fires more lightning]
[She then uses her lightning to throw a screaming Ventus out the window]
Ventus: [falling out of the window] AHHHHHHHHH!!!!!
[Nightmare Moon cackles evilly as Ventus falls to his death]
Twilight Sparkle: What have I done?
[Nightmare Moon pulls up her crown and turns to her]
Nightmare Moon: You're fulfilling your destiny, Twilight. Become my apprentice. Learn to use the Dark Side.
Twilight Sparkle: [exhausted] I will do whatever you ask.
Nightmare Moon: Good!
Twilight Sparkle: Just help me save Flash's life. I can't live without him.
Nightmare Moon: To cheat death is the power only one has achieved, but if we work together, I know we can discover the secret.
Twilight Sparkle: I pledge myself...to your teachings.
Nightmare Moon: Good. Good...The Force is strong with you. You will become a powerful Sith. Henceforth, you shall be known as Nightmare...Nova.
Twilight Sparkle: Thank you, my master.
Nightmare Moon: Rise...
[Meanwhile, Zeta Prime senses Twilight's turn to the Dark Side]
[Back at Equestria]
Nightmare Moon: Because the Council did not trust you, my young apprentice, I believe you are the only Jedi with no knowledge of this plot. When the Jedi learn what has transpired here, they will kill us, along with all the Senators.
Twilight Sparkle: I agree. The Council's next move will be against the Senate.
Nightmare Moon: Every single Jedi, including your friends, the Princesses of the Sun and Moon, your brother, your sister-in-law, and Ryan F-Freeman, is now an enemy of the Republic.
Twilight Sparkle: I understand, Master.
Nightmare Moon: We must move quickly. The Jedi are relentless. If they are not all destroyed, it will be civil war without end. First, I want you to go to the Jedi Temple. We will catch them off balance. Do what must be done, Lady Nova. Do not hesitate. Show no mercy. Only then will you be strong enough with the Dark Side to save Flash.
Twilight Sparkle: What about the other Jedi spread across the galaxy?
Nightmare Moon: Their betrayal will be dealt with. After you have killed all the Jedi in the Temple, go to the Cyberfar system. Wipe out Viceroy Kaos and the other Separatist leaders. Once more the Sith will rule the galaxy! And... we shall have... peace.
[Nightmare Moon turns away from her]
[We see Twilight marching upon the Temple with an army of Equestrian Troopers]