Princess Celestia wrapped up the amber unicorn in her wings and held her tightly.
"I'm so sorry!" Sunset bawled. "It was wrong what I said. I'll never to try leave again, I swear!"
"Shhh," the princess whispered. "Everything's alright. You're safe now."
"No I'm not! Neither are you! Nopony is safe! We have to do something!"
Celestia looked gently into her favorite student's eyes. "Sunset, we're always doing something. Even if that something is nothing. Silence can speak volumes, and sometimes the best choice is to choose to do nothing at all. Sometimes action, even with the best of intentions, can come with horrible and unexpected consequences."
Painful crunching noises erupted from Sunset Shimmer's neck as she struggled to lift her head far enough to gaze with her one remaining eye into her teacher's face. At least the healing magic had stopped most of the bleeding.
"And so you seriously want to do nothing?" Sunset cried. "An entire world, destroyed! An entire people, subjugated and reduced to nothing but mindless thralls for her whims! And it all our fault! It's your...your..."
"My fault?" Celestia smiled sadly. "Perhaps. But right now, you should rest."
Lighting up her horn, the princess redoubled the healing spell on her student, and delivered her unto a dreamless sleep.
"Guards, please take Sunset to her room in the east tower. See that she is not disturbed."
"At once, your majesty."
After the guards carried Sunset away, Celestia paused to take one last look at the mirror in front of her. "Please forgive me, dearest sister," she whispered.
Then she lifted a heavy hoof, and shattered it, forever.