This is how Unicron's return goes in My Little Pony Transformers Prime: The Return of Harmony.
[We begin at Ponyville Maze]
[In the Earth's core]
[Thunderwing approaches and kneels before Unicron]
Thunderwing: What is thy bidding, my master?
Unicron: There is a great disturbance.
Thunderwing: I have felt it.
Unicron: We have a new enemy. The young mare who is the wielder of the Element of Magic. I have no doubt this Alicorn is the fourth Princess.
Thunderwing: How is that possible?
Unicron: Search your feelings, Thunderwing. You will know it to be true. She could destroy us.
Thunderwing: She's just a regular old alicorn.
Unicron: The Alicorn magic is strong with her. The Alicorn of Friendship must not become a princess.
Thunderwing: If she could be turned, she would become a powerful ally.
Unicron: [intrigued] Yes... She would be a great asset. Can it be done?
Thunderwing: She will join us or die, master.
Unicron: Good. And one more thing.
Thunderwing: Yes, my master?
Unicron: Create chaos in Equestria. And grab the Element of Magic for me while you're at it.
Thunderwing: Yes, my master.
[Thunderwing flies off]