Chapter-7: A vow.
After Twilight gets the book, Sombra tells her to open it to page- 43. She does that. He tells her that after he says the vow, she will touch their horns together until his horn glows. He says- I, King Sombra promise to not intentionally destroy Equestria. If I do so, then upon me shall be destruction.” After Sombra says the vow, Twilight makes her and his horn touch. After 10 seconds, Sombra’s horn glows and she moves her horn.
Sombra asks her- “Now will you take me out of here?”
Twilight replies- “Yes, after I finish looking for more things.”
Sombra wants to get out of his room as soon as possible, so he tells her- “Can you see that lever on the wall? Press down on it.”
Twilight presses on the lever, and the wall moves, revealing thousands of crystallized ponies. She is shocked again and asks him- “Did you turn all those ponies to crystal?”
Sombra replies- “Yes.”
Twilight asks- “How can I turn them back to normal?”
Sombra answers her question- “I think I have a book on that. Look on the bookshelves for a book named- How to turn crystallized ponies back to normal."
Twilight searches for the book, and finds it. Sombra tells her to read it. She reads it, and after she reads it says- “This spell needs two Alicorns to do it. I’ll ask Cadence for help.”
Twilight is about to leave when Sombra tells her- “Put my horn in your bag. I don’t want to be found by anypony else.”
Twilight does that and then runs up the stairs, to Cadence. She finds Cadence in her study room.