Twilight learns of Sombra's past, and finds him alive but weakened. She decides to learn how to use his Dark Magic for Good, by taking him back to Ponyville.
As an ancient alicorn trapped inside a library, Princess Twilight spent many a century dreaming of being rescued from her prison. Now, finding herself in an entirely new Equestria, she realizes she'd never planned for somepony to actually succeed...