This is how Thunderwing meets the Alicorns goes in My Little Pony Transformers Prime: When Worlds Collide 3.
[Princess Celestia, Princess Luna, Princess Cadance, and Twilight Sparkle try to blast Thunderwing with magic when Thunderwing freezes them in place and walks over to them]
Thunderwing: The Alicorns I've heard so much about. The Dark Spark. [aims his wrist gun at them] Where is it?
Princess Celestia: We don't know.
Princess Luna: Never seen it.
Princess Cadance: We don't even know what it does.
Twilight Sparkle: We're being honest.
[Thunderwing then senses something]
Thunderwing: The Dark Spark's power. You all have seen it.
[Two Terrorcons arrive with the Dark Spark]
Terrorcon: Sir, we have the Dark Spark. [throws the Dark Spark to Thunderwing, who catches it]
Terrorcon #2: But Autobots inbound. We need more troops.
Thunderwing: Pull the division out. Forget the Autobots. We have what we need. [imprisons the Alicorns in a cage made of Dark Energon and levitates it as they retreat]
[Shining Armor sees Thunderwing with his wife and sister and runs to stop the Follower of Unicron escaping]
Shining Armor: Hey. Stop in the name of the Royal Guard!
[The shuttle door closes and the shuttle takes off]
Shining Armor: [to Optimus] He took Celestia, Luna, Cadance, and Twilight. He took the princesses, they're gone.
Optimus Prime: Yeah. I know.