Human Twilight and Equestrian Twilight turned to face their foe. Dark Twilight looked at them, and then laughed.
"Well done, Twilight Sparkle. But you made one mistake. When you banished your equivalent of me from your mind, you shattered its personality - but not its power."
Equestrian Twilight gasped. "Oh no..."
"What? What did I do?" Human Twilight asked in concern.
A mist of darkness rolled in. Dark Twilight lit her horn, and the darkness spiraled inward towards her horn, like water going down a drain.
"Quick, get her!" Equestrian Twilight screamed, and both Twilights unleashed a massive magical blast.
Dark Twilight was flung away - but the dark mist rushed to her horn, and disappeared.
"Come on, we've got to finish her!" Equestrian Twilight hollered, and Human Twilight swung herself up onto her counterpart's back. "Hang on!"
Equestrian Twilight spread her wings, and took off, soaring high above the 'ground' below. Human Twilight noted that the library had faded away, replaced by a featureless plain of granite.
Dark Twilight stood below them, and unleashed a beam of black-purple energy. Equestrain Twilight dodged, and then pulled into a steep dive, her horn charging up. Human Twilight placed her hands in front of her and concentrated, her hands glowing a vibrant violet.
Twin blasts of destruction hurtled towards Dark Twilight, who spread her wings as the blast front hit. A massive plume of dirt headed upwards, mushrooming out towards the 'sky'.
"Did we get her?" Human Twilight asked.
"AARRRGGGHHH!!!" Dark Twilight bellowed as she rocketed towards their position, encased in a sheath of black-violet energy. Equestrian Twilight generated a shield spell, saving them from destruction, but causing them to fly upward, hitting the 'ceiling' of the 'cavern'.
"Grrr," was Equestrian Twilight's response. She then pushed off, speeding like a bullet at Dark Twilight.
Human Twilight then did something that made Equestrian Twilight question her sanity: she jumped off.
"WHAT ARE YOU DOING??!!!" Equestrian Twilight cried in surprise, unable to stop her charge at Dark Twilight.
Human Twilight did not answer her, and closed her eyes.
Dark Twilight cackled. "Suicide, Twilight? Really? I thought that you were better than that."
Then, as Equestrian Twilight continued her onward rush, Human Twilight snapped open her eyes, and a thermonuclear warhead appeared underneath her, perfectly visualized thanks to her extensive knowledge of physics and chemistry.
Equestrian Twilight came on like a runaway freight train, and impacted Dark Twilight, who shielded herslf but was knocked to the 'wall' of Twilight's mind.
"Is that the best you can do?" Dark Twilight taunted, spreading her wings in preperation for takeoff.
"TAKE COVER!!!"
Dark Twilight snapped her head around, looking at Human Twilight.
She saw a projectile of some sort, hurtling towards her. "What?"
The five-megaton nuke detonated, and Dark Twilight was hurled outwards, the outer barrier of Twilight's mind shattered.
"NOOO!!!"
Human Twilight then jumped up from behind her lead shield, and concentrated, causing a temporary seal to form around the breach.
"You have to seal it!!" she called to her counterpart, who was stepping forward from behind a lead barrier that Human Twilight had visualized.
"I will," Equestrian Twilight promised, smiling at her. "But you need to return to your own body now."
"I know," Human Twilight said. She turned to go, but was halted by Equestrian Twilight once more.
"Yes?" she asked.
"Thank you. For everything."
Human Twilight smiled. "Hey, that's what friends are for." She turned then, and leaped, seeing a brightness as she flew through the air to the edge of Twilight's mind.
Celestia and Luna saw a dark mist shaped like a pony being suddenly ejected out of Twilight's head.
"Now," Celestia said, and she and Luna locked horns. Before the darkness could get its bearings, a shield of gold and blue materialized into existance around it, trapping it.
The darkness pounded furiously at its newly-created prison, but was unable to make so much as a dent.
Celestia and Luna's horns glowed once more, and the sphere began to contract.
The darkness thrashed around furiously, but still could not escape. The sphere continued to shrink, and eventually shrank to the point of having a radius of zero, obliterating the personality known as Dark Twilight forever.
Celestia sighed in relief, and turned to the sleeping ponies on the floor. "Well, that is one crisis averted."
Luna stood next to her, and added, "Now all that remains is the clean up."
Celestia looked at her, then stated, "And the explainations."
Luna nodded her head in assent, then asked, "How much do you wish to tell them?"
Celestia answered, "I do not feel that we can tell them everything. I still cannot go a day without thinking about how she died."
"She is not dead yet," Luna reminded her sister.
"She might as well be," was Celestia's response, and the two stood there quietly, thinking of their shared loss.
Luna broke the silence. "Twilight Sparkle might be able to find a way."
Celestia sighed once more. "Yes, she might - but if I inform her, then her friends will eventually learn of it - and you know that the protection spells we set up around her will kill them. Twilight will be able to pass safely, but they will not - and we did set up the spells to be as close to impossible to undo as we could." Then she paused. "Although there might be a way..." She shook herself. "Now is not the time, Luna. We must take care of our various responsibilities."
"Agreed," was Luna's reply, and they turned their thoughts back to the tasks at hoof once more.