Here's how Nightstar is being controlled by Poultrion in Ernie the Dragon Hunter
[Nighstar lands and Yuna hops off]
Yuna: You can’t do this.
Ernie: Oh, who are you? The Princess of all dragons? [chuckles] I know who you are. Your Princess Yuna, daughter of Princess Luna and aunt Princess Celestia.
Yuna: That’s right.
Ernie: [chuckles] So you think you're gonna stop me?
Yuna: No, let me show you that dragons are….
Ernie: NO!!! Don’t show me anything. Because I have my own dragon. POULTRION RISE!!!!!!!!!!!
[Water guises up and Poultrion comes out!]
Vinny: No way!
Brian: A bewilderbeest!
Yuna: [to herself] Mama was right, there is a bewilderbeest. [out loud] How did you get one?
Ernie: [scoffs] It's simple, my family and friends were taken and eaten by these creatures. So, I took it as a hatchling. I tortured and manipulated it. Till finally it came to my trust. But, better not. I think our little game ends here. Poultrion, show her what control does.
[Nightstar is then shaking over Poultrion’s power]
Yuna: What’s wrong?
Ernie: Don't you get it? It's an alpha dragon, nothing can resit it! Not even your precious night fury!
[Then, Nightstar is now under control of Poultrion]
Princess Luna: [sees what’s happening] Yuna!
Princess Celestia: [gasps]
Ernie: Kill her.
Yuna: What did he just said to you?
[Nightstar is coming closer to Yuna]
Yuna: Hey, come on! Stop! Snap out of it!
[Emperor Lunarlight is running towards them]
Yuna: Nightstar! No!
Nightstar: [builds up fire]
Emperor Lunarlight: Yuna!
Ernie: [evil snickers]
Emperor Lunarlight: YUNA!!!!
Yuna: [gasps] NO!!
[Emperor Lunarlight pushes her out of the way and he’s blasted!]
King Solar Flare, Celestia, and Luna: [gasps]
Ernie: [shocked] Heh.
[“Stoick saves Hiccup” starts playing]
Yuna: No! [uses her magic to clear the ruble] Lunarlight!
King Solar Flare: Father!
Princess Luna: Grandpa, no.
Princess Celestia: [checks for heart beat but hears nothing] He's... [sheds a tear] He’s gone.
Nightstar: [is snapped out of control]
[Everyone comes up to see Lunarlight]
Nightstar: [tries to comfort King Solar Flare]
Yuna: No! Get away! Go on, get out of here! Go away!
Skyla: Yuna, it won't help! She's only...
Yuna: She killed him!
[Nightstar runs off]
Poultrion: [roars and all of the dragons are now under controlled by it]
Ernie: [laughs] [then takes out a Dark-Dragon Ball]
Sideshow Bob: What’s that thing?
Ernie: A Dark-Dragon Ball. It allows me to capture dragons. [throws it at Nightstar] And I always wanted a Night Fury. [throws her back out] 'round up the men, and meet me at Berk! [hops on Nightstar]
Princess Luna: [grabs her] It's too late.
Poultrion: [gets back in the water as Ernie and his crew depart to Berk]
[It later fades to a funeral for Emperor Lunarlight]
Princess Luna: [sees Yuna isn't there] Snowdrop, where’s Yuna?
Snowdrop: She's in our room.
[We see Yuna crying in her bed]
Princess Luna: Hey, there sweetie.
Yuna: Hey Mama. This is all my fault, and Nightstar is gone because of me. I didn’t mean to yell at her I was just mad at her. [cries]
Princess Luna: [puts her wing on her daughter] Shhhh… there, there.
Yuna: We got to get her back and the other dragons! I just don’t know how! [press her head on Luna’s side and muffled cries]
Princess Luna: You see Yuna, when me and Tia were kids we always fight.
Yuna: [calms down] You do?
Princess Luna: Yes. When something goes wrong, I usually blame it on Tia but usually it's my fault. And I told my father the truth. And what Nightstar did wasn’t her fault, it was Ernie’s.
Yuna: That was true.
Princess Luna: Yes, and we're gonna get her back. My sweet, dragon rider.
Brian: Exactly. Because we're going after him with everything we’ve got.