This is how Thunderwing's return goes in My Little Pony Transformers Prime: Optimus' Kingdom.
[Thunderwing runs into Rare Find]
Rare Find: Are you friend or foe?
Thunderwing: "Is he friend or is he foe?", the pony wonders. I can assure you... I am no friend. I am Thunderwing, and I will take what should have been mine long ago.
[Thunderwing drains Rare Find's magic, causing his eyes to become colorless and his cutie mark to vanish. Elsewhere, Celestia wakes up with a start]
Princess Celestia: [gasps]
Princess Luna: Sister, are you alright?!
Princess Celestia: I've just had the most terrible dream.
Princess Luna: Why do you think I'm here? You know as well as I that this was not a dream, but a vision.
Princess Celestia: Then we haven't much time. The stronger he becomes, the more we are all in danger.
Princess Celestia: [narrating] Thunderwing and his brother, Darkblade, came here from a distant land, intent on stealing Equestrian magic. But Darkblade soon came to appreciate the ways of Equestria, even befriending a young unicorn wizard.
Princess Luna: [narrating] Darkblade urged his brother to abandon their plans. When Thunderwing refused, Darkblade alerted us to Thunderwing's intentions.
Princess Celestia: [narrating] Darkblade returned to his own land, and Thunderwing was sent to Tartarus for his crimes. But it appears he has found a way to escape.
Princess Luna: We believe it happened when Cerberus left his post at the gates.
Twilight Sparkle: But that was a long time ago. Why is he just now starting to steal magic?
Princess Celestia: His time in Tartarus left him very weak. He has just now gained enough strength to use his dark powers.
Princess Luna: But with each passing moment, he grows stronger still.
Princess Cadance: And I know just the prince who can stop him.
Optimus Prime: Yes. I'll find him and–
Princess Celestia: No, Optimus. I'm afraid I must call in another to stop Thunderwing. Pythor.
Twilight Sparkle, Princess Luna, Princess Cadance: [gasps]