This is how Von Nebula explains his plan goes in A Canterlot Wedding Part 2.
[Previously on My Little Pony Transformers Prime: Friendship is Magic]
Twilight Sparkle: "Princess Celestia cordially invites you to the wedding of Princess Mi Amore Cadenza and..." [gasp] My brother?
Optimus Prime: Whoa.
Shining Armor: Twily! Ah, I've missed you, kid.
Twilight Sparkle: How dare you not tell me in person that you were getting married!
Optimus Prime: Twilight! Don't let anger consume your judgement.
Shining Armor: A threat has been made against Canterlot. Princess Celestia asked that I help provide additional protection.
Twilight Sparkle: But I'm still pretty ticked you're marrying somepony I don't even know.
Optimus Prime: Pah-lease.
Shining Armor: Princess Mi Amore Cadenza is Cadance.
Twilight Sparkle: Cadance is only the most amazing pony ever!
Optimus Prime: Seriously?
Princess Cadance: Hi, Shining.
Twilight Sparkle: Cadance, it's me! Twilight!
Princess Cadance: Twilight!
[Cadance and Twilight hug]
[Twilight hears Von Nebula]
Von Nebula: Twilight Sparkle.
Twilight Sparkle: Who are you?
[Twilight releases an embodiment of Von Nebula, which starts moving towards the staff]
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]
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!
Princess Celestia: Von Ness?
Von Nebula: Von Ness no longer exists. I am Von Nebula!
[Everyone stares as Von Nebula laughs evilly]
[My Little Pony Transformers Prime: Friendship is Magic theme song plays]
[My Little Pony Transformers Prime: Friendship is Magic theme song ends]
[Von Nebula grabs his staff]
Von Nebula: At long last, Von Nebula has returned.
Princess Celestia: What do you want?
Von Nebula: Canterlot.
Shining Armor: Royal Guard! Assemble!
Von Nebula: It's no use. No one can hear your commanding voice. And no one will ever think to follow your orders, either. Most ponies have forgotten who I am, which is why I will take this land over and no one will interfere with my plans. [laughing]
Shining Armor: Plans? What plans?
Von Nebula: The plans I have for your sister, of course.
Shining Armor: Don't you dare do anything to my sister, you... [growls] MONSTER!!