"Well, I learned a spell earlier. It was Spherical Prison."
Celestia nodded. "Yes, I know that spell. It was used to stall the sun last time."
"I tried casting it earlier but it didn't work!"
"By itself?"
Twilight wondered what she meant, "No, I was with Starlight."
"No, I mean did you cast it through a medium?"
"Ooh, no."
"It needs an amulet to work through, but I have since forgotten how to make that amulet."
"It doesn't exist anymore?"
"It disintegrated upon use."
"I'll go find an amulet book from the archives then. I'll be right back."
Twilight flew through the castle to get a book called the Unabridged History of Amulets. When she returned to the gardens, Celestia was laying on the ground still protecting Canterlot with her magic. Luna was doing her best to help her sister, but the stress of battling the sun was proving to be too much.
"Celestia! Are you okay?"
Celestia makes a noise to signal some sort of consciousness.
"Princess Celestia, we all need you to stay awake. You're the only one who can protect us. I also need you to point out where the amulet is in this book so we can make it again."
Twilight showed her page by page. The deeper into the book they got, the harder it was for Celestia to open her eyes to look. Then came the moment no one wanted.
"Was this the one? The "Amulet of Fire Resistance"? Celestia? Celestia!"
Luna tried to wake her up with a spell that ends a deep slumber, but it did not work. Celestia's horn became inactive, abruptly terminating the protective magic. Instantly, an unforgiving rush of heat and pressure acted like an imploding bomb to all of Canterlot.