This is how Twilight frees Von Nebula goes in A Canterlot Wedding Part 1.
[We see Twilight's friends, the Autobots, Celestia, Cadance, and Shining Armor at the receptional]
Princess Celestia: Perfect, girls. No need to rush. Then of course, Cadance will enter.
[They stand to attention]
Princess Celestia: I'll say a few words, and then we'll begin with the vows. Shining Armor, you'll get the ring from your best mare.
Spike: [deep voice] Do you? [high voice] I do!
Shining Armor: Hey... has anypony seen Twilight?
[Meanwhile, Twilight walks]
[Twilight sees Von Nebula's Black Hole staff]
Von Nebula: (voice) Go on. Do it. Free me from my prison.
[Twilight listens and releases an embodiment of Von Nebula, which starts moving towards the staff]
Applejack: Whoa Nelly. What's Twilight doing?
Rainbow Dash: I dunno.
Rarity: And who is that?
Fluttershy: Some robot, I guess.
Pinkie Pie: Why is he going towards that staff thingy.
Princess Celestia: [gasps] Oh, no.
[The embodiment of Von Nebula reaches the staff and shatters the orb on it and the embodiment disappears and the room starts shaking]
Applejack: Whoa Nelly! What's going on?
Rainbow Dash: We don't know.
Rarity: Must've happened when the staff got touched.
Fluttershy: By that robot.
Pinkie Pie: And what's happening now?
Princess Celestia: An acient evil is being released.
[The shaking continues as two black clawed hands come out of the staff and pull the rest of the body out. The shaking stops and the smoke clears and everyone sees a black figure to be none other than Von Nebula. Von Nebula grabs his staff and repairs the orb and looks at Twilight's friends, the Autobots, Celestia, Cadance, and Shining Armor]
Von Nebula: At last! I am free!
Rainbow Dash: [gasp]
Pinkie Pie: [gasp]
Princess Cadance: [gasp]
Shining Armor: [gasp]
Princess Celestia: Von Ness? Is that you?
Von Nebula: Von Ness no longer exists. I am Von Nebula!
Von Nebula: That's right! You have every right to be afraid.
[Everyone stares as Von Nebula laughs evilly]
[To be continued...]