Since her childhood, the Amyrlin has shielded Elayna from the One Power for her own protection, and the odd ter’angreal is able to unlock that power. Just as the Amyrlin is about to use the ter’angreal on Elayna, the assassin and Sephraem break in, kill the Amyrlin, and take the ter’angreal as well as the Amyrlin's seal.
The Amyrlin has kept Elayna from tapping into the One Power since childhood for her own safety, and a rare object can unlock that power. Just as the Amyrlin is about to use the object on Elayna, assassins and Sephraem burst in, kill the Amyrlin, and take both the object and the Amyrlin's seal.