In the darkness, somepony coughed.
"Who was it?"
A dim violet light sprang into being. Twilight Sparkle's horn barely illuminated the cramped, windowless room and the six ponies within. A world of dark purples and inky blacks sprung into being, colors distorted by the monochrome aura of the magical implement. Shadows pooled in the corners, lurking just beyond the reach of the unicorn's spell.
Rainbow Dash watched as Twilight glared at each of their friends in turn. None spoke. Silhouettes danced along the walls as the unicorn moved her head, causing the light to jerk and shift. Pinkie Pie huddled near the barred door, eyes wide and uncomprehending as she stared off into an uncertain distance. The baker twitched slightly, squeezing the bag of flour that she clutched so tightly to her chest. On the room's only bed, Rarity lay next to Applejack, her head buried in the farmer's shoulder. The dressmaker shook as she sobbed quietly. Dash didn't know whether the unicorn's distress was because of their situation or because of their hygiene; the room reeked of unwashed bodies, and Rarity's mane, like everypony else's, was tangled and knotted and hung heavy with filth. Applejack, for her part, merely stared stoically back at Twilight Sparkle when the librarian's gaze turned to her.
And then those eyes were on Dash, and on Fluttershy. The two were locked in an embrace; it was the only comfort they had in the room. The weather pony found her voice when the other pegasus began to shudder, "What does it matter?"
Twilight Sparkle's brow furrowed, "Get serious, Dash. It's better for five of us to get through this than none of us. Coughing is the only warning we get."
Fluttershy's whisper carried well enough in the enclosed space, "Couldn't we...um... wait and see? It could just be dust or a cold. Please?"
"I really wish that we could risk it, Fluttershy," Twilight said, the sharp edge of her voice softening slightly, "But there's no going back once it reaches the final stage. You saw what happened out there. Do you really want that in here?"
Applejack sighed, then spoke up, "She's right sugar cube. It's not contagious until the end. We got a choice to make -one death, or six."
Rainbow Dash gritted her teeth, "Isn't there anything that can be done? Can't your magic cure it? Stop it? Slow it down?" She paused, then added in a hoarse whisper, "Anything?"
The unicorn shook her head, "Even the Princess can't cure it. She can only contain outbreaks that do happen. Every carrier has to die. That's the only way the rest of us can be safe. The only way the rest of Equestria can be safe. You know that, Dash. Time matters here; why are you stalling?"
The weather pony's mouth worked, but no words came forth. Fluttershy's voice, suddenly confident, filled the gap, "I coughed. It was me." Rainbow Dash gaped at the other pegasus, eyes wide with shock and horror. The pink-maned mare turned to look at her and spoke, once again hesitant, "It's okay. It'll be okay. I couldn't live with myself if any of you died, and I could have prevented it. It's better this way."
Dash shook her head slowly, and whispered a single, "No."
Applejack's voice was grim, "Her mind's made up. It's a tough choice, but Fluttershy's got the strength of an ox, on the inside."
"H...how can we... how is she going to...?" The pegasus trailed off.
Twilight Sparkle sighed and slumped to the floor. "We don't have any weapons, and I'm not trained in combat magic. Even if I were, it can't be me. It has to be you, Dash."
"What? Why me?"
The unicorn looked away, "I know you well enough to realize that you'd never just stand aside and let one of us lay a hoof on your fillyfriend. Maybe intellectually you'd know that it's necessary, but your emotions get the better of you. Ask yourself, if it has to be any of us, would you really want anypony but yourself to... to..."
Rainbow Dash rose to her hooves. "You're right." The mare's voice was hollow. She took a step away from the other pegasus.
Applejack spoke, "Fluttershy, sit up and put your head up against the wall. It'll be a mite... a mite...," the mare choked back tears, "...quicker."
"I'm so sorry, Rainbow Dash. I wish it could be any other way," Twilight said.
The rainbow-maned pegasus looked over her shoulder to meet the eyes of the mare she loved. Fluttershy had taken the Earth pony's advice. They stared at each other silently for a moment. Eventually, Dash found words, as insufficient as they were, "I'm sorry. I love you. I'm so sorry."
The other pegasus replied, voice shaking, "I love you too. And I forgive you. It's better this way. Just... make sure somepony takes care of the animals when this is all over. When everything is back to normal."
Rainbow Dash whispered, "I promise." She took careful aim. One buck. It would be quick. Fluttershy deserved that much, at least. When she was sure she'd connect, vision or not, she spoke, "Cut the light, Twilight. Nopony should have to see this." The horn's glow disappeared.
Years of running and kicking clouds had honed Dash's muscles. She reared up on her front legs, gathered all her strength, and kicked out. She connected, and a dull thud was accompanied by the sensation of warm droplets splattering across her flank. It was followed by a high-pitched moan from the agonized pony behind her.
Rainbow Dash reacted to Fluttershy's continued suffering without thought. She reared again and bucked repeatedly, steadying herself and adding force with swift flaps of her wings. On her fourth kick, the other mare's keening stopped with a sickening crunch.
The weather mare's hooves dropped back to the floor. She stood shaking, her eyes wide and her heart beating nearly out of her chest. Her fetlocks were drenched with sticky blood; she could feel it beginning to clot. She wondered despite herself whether the mess would stain, whether her coat would always bear the red residue of her deed. She wept quietly to herself.