Double Rainboom

It was a strangely quiet day in Ponyville, and ponies were taking advantage of that. Some were gardening, some were baking, some were just enjoying the sunshine, but others were spending their days differently.

Deep beneath the Golden Oak Library, head librarian and part-time savior of pony-kind Twilight Sparkle was working in front of a small table, surrounded by various brightly colored liquids and sludge, as well as a book covered in brand of chicken-scratch. She slowly levitated a beaker of pink liquid from behind her and began mixing it with other fluids before hearing a knock at the front door. "Spike! Can you get that?" She bellowed only to be met by silence. "Oh right, Rarity has him for today." She heaved a slightly annoyed sigh before setting down her beakers, and leaving her basement to open the door. On the other side of the door was a familiar blue Pegasus.

"Sup?" She said casually as she walked into the library.

"Oh. Hey Rainbow. What brings you here?" Twilight asked, standing aside so her friend could walk in.

"I was wondering if you had the second Daring Do in stock." She replied as she began looking through the shelves.

"I'm pretty sure it's in." The unicorn said, beginning to move towards the basement door. "You take a look. I need to attend to something." The Pegasus nodded before poking through the books on the shelves, leaving the unicorn to her own business. That lasted for about 10 seconds before she heard the basement door fly open violently. She lurched forward, nearly smacking her head into the books as she turned towards the door. She rushed down the basement stairs to see Twilight flat on her back and levitating a vial of purple ooze.

"Twilight!" Rainbow yelled, racing to her friend's side. "Are you okay?"

"Yeah, I'm fine." The unicorn groaned as she got to her hooves.

"What was that?" The Pegasus asked, as she looked around the decimated room.

"A little potion I've been working on for quite a while." She said with a proud expression.

"And what does it do?"

"It's… a talent-enhancer." Twilight replied, trying to find the right words. "It boosts the natural abilities of the pony who drinks it."

"And you made it, why?"

"Hey. Everyone needs a hobby. And being the librarian of the quietest library in Equestria gets boring." The Unicorn replied, beginning to walk back upstairs. Rainbow spent a good 5 seconds thinking about what she could do if she drank that potion, and she liked the ideas.

"Can I try it?" She asked, following Twilight and looking hopeful.

"Not yet." Twilight replied. "I'm not sure it works right yet, and I don't want to poison one of my friends."

"Come on Twilight." Rainbow replied. "I've been needing to stretch my wings since I got out of the hospital, and I put enough trust in you to think that what you made won't kill me."

"Well, okay." She said, still looking unsure as the two emerged outside. "But I think that before we test it, we should move away from Ponyville."

"Sure. You're the mad doctor." Rainbow muttered as the two galloped away from Ponyville.

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The two arrived at a hill overlooking not only Ponyville, but Canterlot, the Everfree Forest, and even as far as the deserts to the south. "Now, first things first." Twilight said as she readied a quill and notebook and gave the Enhancer to Rainbow. "Bottoms up." The Pegasus only nodded before drinking the contents of the vial. "So, what does it taste like?"

"Grape." Rainbow said after a few seconds. "Definitely grape."

"Now, I want you to gently get yourself off the ground." The unicorn instructed, notebook at the ready. Rainbow once again said nothing as she flapped her wings gently and rocketed into the air. She stopped herself, hovering in midair.

"What's next!?" She yelled to the ground.

"Go to Canterlot, circle around the mountain and come back." Twilight replied. The Pegasus nodded before zooming towards the city in the mountain, circling around it and coming back. "Wow. 20 seconds. Impressive." She said as Rainbow landed next to her. "Now, one last test." Rainbow nodded, feeling a bit more cocky than normal.

"Sure. What do you need?" The Pegasus asked, tapping her hoof against the ground.

"I want to see how fast you can achieve a Sonic Rainboom." The Unicorn said, glancing up at the sky. "Start at as high up you're willing to go, and fly directly towards the ground."

"You got it." She said with a cocky grin, beginning to fly straight into the sky before stopping at 15000 feet. She took a deep breath before beginning to fly towards the ground. Within 10 seconds of her descent, she achieved the Rainboom, though she didn't stop. She kept moving, only now the rainbow trail to begin to consume her beginning with her back legs.

"What's she doing!?" Twilight yelled to herself. "She's already done the Rainboom! Why isn't she slowing down!?" She looked towards the streak of color in the sky, feeling dread form in the pit of her stomach. "Hey! Rainbow Dash! Slow down!" She yelled, but the Pegasus couldn't hear her. Her friend's voice was being drowned out by the wind rushing past her ears, the adrenaline coursing through her, and the fear that she wouldn't be able to pull up in time at this speed. Right before Rainbow Dash crashed into the ground, a loud cracking noise could be heard throughout the area. Twilight looked up, now seeing two Rainbow-Halo that accompanied the Rainboom, but there was two of them, neither one appearing to overlap with the other.

"What just happened!?" Twilight yelled, feeling a strong gale nearly knock her off her hooves and blind her for a moment. After a few seconds, she opened her eyes, seeing a large crater at the base of the hill. "Rainbow!" She bellowed as she ran down the hill, only to see there was nothing in it, except for a large, white crack in the ground that quickly sealed itself. "What was that?" She sat there and pondered the question for a few minutes as the Rainbow-Halos dispersed and faded.

"Twilight!" A thick southern accent bellowed from behind her as the rest of her friends ran to her side. "Twilight. What happened?" Applejack asked as Pinkie, Rarity and Fluttershy looked to the sky in awe of what remained of the two halos.

"Rainbow and I were testing something I had made, and she just, disappeared." Twilight replied, her eyes turning to the crater.

"And where is she now?" The apple farmer asked, now looking at the crater.

"That's the worst part." The Unicorn replied, worry apparent on her face. "I don't know."