This is how Tempest reveals Shadow Mane was her student goes in Ghost Story.
[The ponies, Autobots, and Tempest sit round a camp fire]
Tempest Shadow: I have something to tell you all. Something I should've told you a long time.
Bulkhead: Is this about how the Storm King hired you to be his lieutenant? Or is it about Shadow Mane?
Tempest Shadow: Shadow Mane may have been my first student, but he was also my greatest regret.
[A flashback to when Tempest worked for the Storm King]
[She places a spare down]
Tempest Shadow: (voice over) Before he was a successor of the Pony of Shadows, it wasn't I who found him.
[A young Shadow Mane comes over]
Tempest Shadow: (voice over) But he who found me. My partner Grubber had just gone off to find more Storm King troopers, which gave me time to more teaching. Shadow Mane was the perfect student.
[Tempest gives him a bread piece]
Tempest Shadow: (voice over) He did everything I ask him to do.
[The other students beat up Shadow Mane]
[Tempest comes over]
Tempest Shadow: Get up.
[He does so]
Tempest Shadow: (voice over) I had shown him things he had never seen before.
[Tempest teaches him to petrify objects]
[Shadow Mane tries it on a bee]
[It turns stone solid]
Tempest Shadow: (voice over) And he had shown me a few things too. He had a power over the shadows I had never seen. He was a descendant of a pony who had previously brought the Pillars of Old Equestria together. It was then I thought I had found the One. Perhaps he would be the savior of Equestria when the time to launch our attack came.
[Rainbow Dash interrupts]
Rainbow Dash: Whoa, whoa, whoa, back up. You're saying Shadow Mane, the successor of the Pony of Shadows, was gonna be the savior of Equestria?
[The wind blows slightly]
Smokescreen: Did that just happen? Maybe I don't like what we're going through.
Pinkie Pie: What happened next, Tempest?
Tempest Shadow: After I told him he could be the savior of Equestria, there was a hunger outbreak.
[Back in Tempest's flashback, Shadow Mane bucks a student]
[He blasts two others with his magic]
[He then pounces the final one]
Shadow Mane: Get up. Get up!
Tempest Shadow: Enough.
Shadow Mane: But Tempest, if I'm gonna be the savior of Equestria, I need all the...
Tempest Shadow: I said enough!
[Later, Tempest is in her quarters]
Tempest Shadow: (voice over) There was a problem. I feared I had made a terrible mistake. But it wasn't for me to decide. It was destiny.
[Tempest is later seen with the Elements of Harmony]
[Shadow Mane enters]
[He waits for the Elements to respond but they don't glow]
Tempest Shadow: (voice over) When the Elements didn't respond like they did to Twilight later, I knew Shadow Mane wouldn't become the Chosen One.
Shadow Mane: I'll train more. Learn more lessons.
Tempest Shadow: I'm sorry. Destiny has spoken.
Shadow Mane: Then I refuse to listen! I am the savior of Equestria! You made me believe!
[Shadow Mane runs off in anger]
[Tempest looks down. Shadow Mane is later seen packing his saddlebags and placing them on his back before walking away from Tempest]
Tempest Shadow: (voice over) He told me that he would go looking for Starswirl's journal to prove he was worthy. I told him it couldn't be found. He left no message. I had left the gate open in hope Shadow Mane returned, but he never came back.
[The flashback ends]
Tempest Shadow: I am saddened he was banished to Limbo, but what worries me more is that he escaped and for what reason.
Optimus Prime: [punches a rock in rage, shattering it]
[The others look at him]
Optimus Prime: What?
Fluttershy: Why are you so angry?
Ratchet: Is this about Twilight?
Optimus Prime: Yes.
[Fluttershy and Ratchet comfort him]
[Optimus starts to cry softly]
[Fluttershy looks down then up at the sky]