A/N: Thank you Laureleaf for your review!
Based on a prompt from an anon on tumblr. Season 10, AU in that Cas and Charlie met earlier in the season and not right before she, you know, was murdered.
"Fight the Fairies"
Castiel used his key to let himself into the bunker. After yet another dead end lead on the Mark of Cain, he was feeling rather dispirited. He was a celestial being millennia old, he should be able to help his friends with anything they might be dealing with. But he was failing in this. And the longer Dean bore the Mark, the more worried Castiel was for the man's soul. He had saved the Righteous Man once from Perdition; it tore him apart to face losing him this way now.
His steps were heavy as he descended the stairs into the war room, the reverberations clunking dully throughout the otherwise silent bunker. It was very silent, actually. Perhaps the Winchesters weren't home. Castiel felt a pang of loneliness at that.
But as he walked into the library, he spotted a set of flannel clad limbs sprawled on the floor.
"Sam!" Castiel darted over and dropped down next to the unconscious Winchester, searching for injury. He didn't see any. The man appeared to be just unconscious.
Castiel was about to shout for Dean when he spotted two more bodies on the floor—Dean and Charlie. He surged to his feet and rushed to their sides, checking them over as well. They were alive, and seemingly unharmed, save that people didn't just drop like this at the same time. Castiel swept his gaze around the room. There were no signs of a struggle, but there were some open books on the table, and he could see some circular looking rune painted on the wood. They had been doing something that had obviously backfired.
Castiel pressed two fingers to Dean's forehead, but nothing happened. Frowning, he leaned toward Charlie and tried again. Nothing. He couldn't heal them.
Because this wasn't natural. Something sparkled in Charlie's hair, and Castiel reached out to touch it. The residue was gritty between his fingers and had a floral scent to it. Fairy dust.
That could explain their state now. Fairy dust could be used as an anesthetic. If they had accidentally dosed themselves…
He got to his feet and headed for the storerooms. The only way to counter fairy dust was with an herbal potion.
Castiel paused halfway down the corridor when something clattered down the hallway. He tensed. Who else could possibly be here? He made his way forward cautiously, the noises growing louder like someone was throwing things. The door to one of the artifact rooms was wide open, and the inside looked as though it had been ransacked, objects strewn across the floor and even one shelf unit knocked over. Castiel stepped inside, only to go stock still when he caught sight of a giant blue creature flitting about through the air. He recognized the essence of fairy and the cobalt skin with gossamer dragonfly wings, but it was much too large, about the size of a grown human torso.
The fairy pulled up short mid-flight and hovered in the air, beady eyes narrowing on Castiel. "What's this?" it hissed in a high-pitched voice. "An angel? What's a halo doing here?"
"You're the one who doesn't belong here," he replied tensely. "Did you spell the humans in the library?"
The fairy sucked air between its jagged teeth. "Yes. The silly girl summoned me. Wanted to ask questions about the Mark of Cain. And then the human who bears it… Insolence!" the fairy spat. "I showed them. And look at this treasure trove I found in return!" It giggled and flitted side to side. "So many powerful artifacts to devour."
The fairy snatched up a chalice, and as it held the object, they both began to glow. The fairy let out a groan as its mass stretched and grew.
Castiel dropped his angel blade from his sleeve into his hand, prepared to stop the creature before it could do more damage by consuming power it wasn't meant to contain, but the thing was too fast. It whipped around and flew straight at him, slamming into his chest and flinging him to the floor. Fairy dust sprinkled around him and he coughed as it filtered down his throat.
"No?" the fairy simpered when it didn't have the same effect as on mortals. "Alright, I've learned some other tricks."
A branch of blue lightning shot down from the fairy to Castiel. He rolled to the side, the bolt striking the concrete instead of him. Lurching to his feet, he dove for the door, barreling through and slamming it shut behind him. The fairy banged against it. Castiel put his hand over the handle and heated the metal with his grace until the lock had fused, trapping the fairy within. This was a problem, and he could probably use the Winchesters' help.
So he hurried to the storeroom and gathered up the ingredients to make the antidote with. It was rather simple to mix up, and thankfully all the ingredients were on hand. But as he finished, he heard banging up in the ventilation shafts. Great.
Castiel rushed back to the library, only to pull up short when he found the fairy already there, hovering over his friends. Electric blue squiggles wreathed the fairy's hands.
"Eenie, meenie, miney, mo…" Slivers of lightning spat from its fingertips.
Castiel set the antidote on a shelf and drew his angel blade again. Arching his arm back, he launched it through the air to strike the fairy in the chest. Unfortunately because it'd been moving, the blade didn't pierce its core to instantly kill it.
The fairy shrieked and yanked the blade out, tossing it away. It then flew at Castiel but he dove out of the way. He scrambled for his fallen blade across the room when barbs of lightning hit his back and shot through his body, seizing his muscles. He crashed to the floor in a sizzling, convulsing heap.
The fairy landed on his back, hopping slightly as it grabbed his shoulder and flipped him. Then it dug taloned fingers into his chest.
"Can we devour an angel's power?" it hissed. "Let's found out."
Castiel's back arched as those claws pierced fabric and flesh, all the way down to his grace. A silent scream stuck in his throat from his vessel's lungs seizing. His grace flinched and twisted, but he could feel something attempting to siphon it out. He couldn't breathe, couldn't move. The fairy cackled.
A glint of silver caught his eye. Castiel stretched one hand out across the floor, fingers extended to the max. The angel blade vibrated in response but stayed where it was. Castiel's grace was on fire, being pulled in two directions. Gritting his teeth, he threw every ounce of strength into it. The blade slid across the floor into his outstretched hand, and he stabbed it up through the fairy's throat.
The creature jerked and gurgled in shock. Castiel twisted the blade as he yanked it out and then followed through with a thrust up into the sternum. The fairy shrieked and fell backward, the angel blade still sticking out of it as it writhed on the floor and swiftly shriveled up into an unmoving husk.
Castiel dropped his head back against the floor with a thunk. Every fiber in his body, both human and angelic essence, was shuddering in agony. He lolled his head toward where the Winchesters and Charlie still slept on. They'd be fine for a little bit longer, he decided, and closed his eyes.
He didn't know how much time had passed when he finally woke up. At least the fire in his body had dulled to an all-encompassing ache, and he mustered the wherewithal to push himself upright. The room spun for a moment before he could get to his feet. Then he lumbered over to the shelf to retrieve the reviving potion. Hobbling back over toward the others, he stopped over each of them to splash them in the face with the antidote.
The effect only took a few seconds. Dean sputtered and flailed as he came awake. Charlie sucked in a gasp and blinked in confusion. Sam coughed and sputtered like Dean, looking around in bewilderment.
"Wh- what happened?" Sam asked.
"Apparently you summoned a fairy," Castiel replied, leaning against the wall tiredly.
Sam squinted up at him. "Uh, yeah."
"Where is the little bastard?" Dean demanded.
Castiel gestured vaguely toward the desiccated corpse in the middle of the library floor.
Charlie's brows rose sharply. "It's…bigger."
"It was feeding off the power in some of the bunker's artifacts," Castiel explained.
Sam frowned at him. "You okay?"
He pushed away from the wall and shuffled toward the kitchen. "I need some coffee."