Author's Notes:

This fic is now canceled. You can find a brief explanation in this blog post.

For what it's worth, below is a rough outline of the final chapters.

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Chapter 11; Twilight searches for Pinkie, finds her turning some of the glass windows into candy or similar. After she orders Pinkie to stop messing up her castle, she tries some lasers and stuff but Pinkie just teleports out of the way. Pinkie says she should probably go wake Luna up--the more the merrier. Twilight tells her that’s a bad idea, so Pinkie decides to go wake up Discord instead, then teleports away.

Cut to throne room. Fluttershy is trying to tend to Rainbow Dash’s unconscious form. Applejack berates Fluttershy for causing all this, accusing her of being a terrible friend. Fluttershy gets upset, says she was only trying to help, then barfs out a bunch of yellow goo.

Meanwhile Rarity broke Celestia out of her cage, using one of the discarded spears as a lever and telekinesis and brute force to force the metal apart. Celestia and AJ note that the magic barf must be caused when something contradicts their Element and forces Discord’s magic out of their body.

Rainbow Dash wakes up and still suffers some residual effects from the magic (so she’s still clingy, but is now mortified with it along with Applejack). There’s a ruckus in the garden and Celestia teleports them there.

Chapter 12; All the ponies meet up in the garden, where Twilight and Pinkie are fighting (“there’s confetti and broken statues everywhere”). The ponies manage to convince Pinkie that what she is doing is making everyone unhappy, and she doublebarfs all of Discord’s magic.

Chapter 13; Twilight does another creepy, crazy monologue like the one from the throne room. After all, now that Pinkie’s out of the way, she can get back to killing Celestia.

The other characters have to convince her that they are her friends, and that all this power will drive everypony away. During this "I Know You're in There Somewhere" Fight, Applejack has to lie and Rarity has to be selfish when talking to Twilight, which causes them to barf up their magic too.

Twilight has a battle with her inner monologue/Discord’s voice, trying to reconcile all the power she has with what she would need to do to her friends and mentor. There’s a huge flash of light, and her laser wings flake away as she falls to the ground. Fluttershy and Dash catch her, and she barfs out purple and black goo once they’re on the ground.

Chapter 14; Twilight and Celestia are sitting in the gardens. The sun slowly sets in the background.

“Princess, I’m sorry.”
“You have nothing to apologize for, Twilight. You were not yourself.”
“But, part of me enjoyed it. Having all that power. If I hadn’t been so weak, then Discord’s magic never would have been able to make me do all this.” Twilight turned her head away. “Letting myself be controlled, it makes me just as responsible as him.”
Celestia draped a wing over Twilight. “Don’t be absurd, Twilight. You never should apologize for your positive qualities. Yes, your desire to learn and grow and improve may have made you susceptible to Discord’s trap.” Celestia put a hoof under Twilight’s chin, gently moved her head up. “But it is also what made you my student. It is also what helped you free my sister. All good qualities have their dark sides, as does anything in excess. But they are also what make each pony unique and capable.”
Twilight didn’t make eye contact with Celestia. “Doesn’t it ever worry you, though? Having good qualities so closely related to bad qualities?”
Celestia didn’t answer right away. “Sometimes, yes. I have met ponies who could not balance themselves, whose good qualities prevented me from helping them.” Celestia leaned down. “But you are not them, Twilight. Discord may have guessed at some of your weaknesses, but he underestimated your strength.”
Twilight did not answer. She merely sat there, staring at the sunset. Celestia did not press the question. She simply left her wing there, joined her student in the quiet of the sunset.

A yawn broke out behind them. Gravel and bits of statue crunched under her hoof as Luna walked into the garden. “Sister, did I miss something?”