Here's how telling Sylveon the bad news goes in Revenge of the Nightmare Family & Changling Queen.
[we come to the Eeeveelution house where we see Twilight, Shining Armor, and Cadance's shuttle parked on the balcony]
Sylveon: What are you guys doing here?
Cadance: When was the last time you saw, Brian?
Sylveon: Yesterday. Why?
Shining Armor: Do you know where he was going?
Sylveon: He said he was going to Cloud City to end the war. Why are you asking this of me?
Twilight: Sylveon, we need your help. Brian's in grave danger.
Sylveon: From the Sith?
Shining Armor: No. From himself. [sighs] Sylveon, Brian has turned to the Dark Side of the Force.
Sylveon: [slaps him] You're wrong! How could you even say that?!
Cadance: We've seen a security video, of him... [a tears streams down] killing younglings and the Royal Guards.
Sylveon: Not Brian, he couldn't!
Twilight: Maybe you should listen to this. [plays Judy's carrot pen]
Brian: (on speaker) "Oh, yeah, Judy? You're just a dumb bunny who doesn't deserve to be a cop! And as for you, Blythe! You'd never should have trained the pets with the Force and you give fashion designers a bad name! And Celestia, Luna, Cadance, you made the dumbest decision to sacrifice your magic, and you got banished to Tartarus, hence why you're big, stupid cowards, Hiccup, you're not worthy to be a dragon master, Twilight, you and the others are excuses for their given elements, and you should have dated a handsome stallion, instead of Thomas, and Thomas, you shouldn't be in charge, I should! And as for.... You! You're too stupid not to invite me to your last stinking adventure!"
Sylveon: Oh, my gosh!
Twilight: Has he always been jealous because of that?
Sylveon: Yes... he was.
Shining Armor: So he was lying to us this whole time. And he was decieved, we all were. Nightmare Moon, King Sombra, Queen Chrysalis, and Discord have returned. With a greater Changling army, one that is much bigger. But the main fact is that after the Crusts and Biskits escaped, Nightmare Moon and King Sombra stormed the castle and Brian became his new apprentice.
Sylveon: [walks toward the edge of a cliff and sits down] I don't believe, you. I can't.
Twilight: Sylveon, we must find him.
Sylveon: [tuns back] You're gonna kill him, aren't you?
Shining Armor: He has become a very great threat.
Slyveon: You can't. [saldy looks down to the cliff]
Cadance: We're so sorry.
[The 3 leave to their shuttle and it takes off to Cloud City. And Sylveon weeps, as Eevee, his brothers, and sisters walked up]
Eevee: What's wrong?
Sylveon: Not now, baby.
[It cuts to Cloud City with Brian staring at the carnage he caused, then after a long stare and a tear streams down his face]
[we return to the others as they continue their way]
[then Twilight appears on the hologram]
Twilight: She didn't take it that easy.
Vinny: Did you found out where Brian is?
Shining Armor: Yes, he's at Cloud City.