Twilight discovers The Facility, and has to make difficult choices. She needs to hold herself firm, while something that thinks faster and better than anything else plots to manipulate her.
Sometimes, a nightmare is something you wake from. Other times, it's something you find yourself living; and when that happens, your only hope to end it is to face it, and pray that it's something you can end.
The first chance was little more than a frail shadow. Now, she has a chance. Now, she can do it again. Ponies need to be saved and Twilight Sparkle is just the pony to do so.
a thousand years ago, Celestia and Luna slew each other. Five hundred years ago, Starswirl set off to try and find a cure to the curse. And now, six mare are chosen to be Equestria's last chance. If they ever figure that out anyway.