This is how Celestia becomes Makuta's queen goes in The Rise of Daybreaker.
Princess Celestia: Hope this works.
[She hears a noise]
Princess Celestia: Huh? What's that I'm hearing?
[A poison vine appears]
Princess Celestia: Ahh! A poison vine! Get away!
[The poison vine sprays her and she starts coughing]
[Celestia coughs five times and begins to feel woozy and starts to pass out]
Princess Celestia: [weakly] I have to.... [coughs] save... [coughs] the Wheel... [coughs] of... Harmo...
[Celestia collapses unconscious as her mane and tail land on the floor, each facing a different direction, and the poison vine recoils]
[The Plunder Vines start wrapping themselves around the unconscious Celestia]
[Minutes later, Celestia wakes up and feels something tugging on her as her mane and tail continue to sway in two directions]
Princess Celestia: What happened? And why am I tied up?
[She suddenly realizes]
Princess Celestia: Oh no! Makuta has set a trap!
Makuta: Yes. Thanks to my Plunder Vines, they got you right where they want you.
Princess Celestia: Let me go, Makuta! You're corrupting the Wheel of Harmony with Dark Energon.
Makuta: Seems I haven't given you enough information about my evil plan. You see, not only do I want to take control of the Wheel of Harmony, but I want to use it to make you my Dark Queen.
Princess Celestia: Dark Queen? You planned this?!
Makuta: Yes, Princess Celestia. Soon you will succumb to the Plunder Vines' Dark Energon.
Princess Celestia: What are you going to do to me?!
Makuta: You'll see.
Princess Celestia: Please, Makuta, let me go!
[The transformation begins]
[The vines close around Celestia]
Princess Celestia: Twilight, Optimus, do something! The Plunder Vines are draining my magic and I can't do anything to stop it!
[The vines start draining Celestia's magic as a yellow aura surrounds her]
[The yellow aura gets brighter and brighter as the vines drain Celestia's magic]
[The vines wrap themselves around her]
Princess Celestia: Hurry! The dark energy from the vines is taking over my body!
[The vines completely enclose her within them]
Princess Celestia: Twilight!
[A flash of light happens]
Princess Celestia: NOOOOOOO!
[The light fades and Celestia's mane and tail turn to flames and her cutie mark becomes a burning sun and her eyes turn dark red with yellow pupils and her hoof guards grow longer turning orange and her crown becomes a helmet and her chestplate turns the same color and her wings grow bigger with more feathers]
Daybreaker: [laughing] Oh yes. I can feel dark energy flowing through my veins!
Makuta: It worked. The transformation worked. I can sense a lot of energy coming from you.
Daybreaker: And it was all thanks to you, Makuta.
Daybreaker: What have we here? The Decepticons. Mind if I use them for evil?
Makuta: They're all yours. Soon, you will be unstoppable.
Daybreaker: So, where do we start?
Makuta: A good place to start is by making daytime last for eternity.
Daybreaker: Just the way I like it. Hot and sunny.
Makuta: This is gonna be fun, Celestia. Wait, I have another name for you. Since you are evil and my queen, I will call you Queen Daymare Sun.
Daybreaker: Queen Daymare Sun? How about Queen Daybreaker?