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 spear 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.
[Tempest gives him a bread piece]
Tempest Shadow: (voice over) 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.
[Shadow Mane takes it and sits down]
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]