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Grace, under the wrong circumstances, could rot.
Perhaps that was an erroneously organic way to put it, but Castiel’s recent brush with the human experience had left him with an awkward preoccupation with biology.
The stolen grace sat heavy in him, decaying by the second, collapsing into itself, burning him up. Once it had consumed itself completely, it would kill him - not in the sudden final surge of light, the way real angels died, but slowly and agonizingly. He laid in bed, feverish and shaking, as it tore into his body viciously. He couldn’t tolerate anything more than a bathrobe on his skin, sensitive to the point of pain.
It wouldn't be long now. A small comfort - Castiel had failed at everything he had set out to do, but maybe he could finally succeed at dying, at least.
A rustle came from behind him and he scrambled into a seating position, reaching for his blade.
“Wow, that's the kind of welcome I get?”
It was Dean. It wasn't Dean. Castiel wasn't sure what was more accurate. Dean's body, and the demon within - sitting on Castiel's bed. Green eyes - deceptively identical, a few freckles drawn out by the sun, a loose strand of hair hanging over his forehead. And a pulsating demonic miasma drawn over his carcass, enigmatic. Not tortured into its shape as most demons were, but instead unnaturally crumpled and twisted. All the strands of Dean’s being oriented towards the Mark, which shone bloody red in the middle, its inevitable gravitational pull swallowing all Dean used to be, bit by bit.
Castiel turned his eyes away.
“I've got a present for you,” Dean sounded eager, hungry. That did not bode well.
“I don't want it,” Castiel rasped, throat suddenly tight.
“You'll want this one.”
Castiel turned to look, feeling obligated. Dean held a small bottle with angelic grace swirling inside. Castiel’s heart fell - there were so few of them remaining, and Dean…
“Whose?” His voice was too thin, even to his own ears.
Dean grinned wider and Castiel frantically searched his memory for an angel still alive whose death could hurt him enough to produce this reaction in Dean.
Dean shook his head. “You have to guess.”
Castiel sobbed and covered his face in his hands. He couldn’t think of anyone - all the angels he had ever called friends were dead. He heard an impatient huff and then Dean was prying his hands from his face.
“You’re supposed to be grateful,” he grumbled. He held Castiel’s hands in his own - his body warm - his palms calloused. It felt like Dean - indistinguishable from the real thing. He pushed Castiel’s hands into his lap and took the vial out of his shirt pocket again - shook it right under his nose, as Castiel felt the urge to turn away.
“Well? Whose is it?”
“I don’t know!”
Dean laughed, achingly familiar, and shook his head. “I can’t believe you don't even recognize it! It’s yours, buddy.”
Castiel froze. The word sounded wrong in Dean’s voice now but moreover - for a second he believed - couldn’t prevent the wild seed of hope taking root in him and he could tell Dean noticed, ever eagle-eyed, drinking in whatever expression he was seeing on Castiel’s face.
“How?”
Dean leaned back against the headboard, looking bashful - satisfied now that he got a reaction out of Castiel. “Metatron is a little bitch. He broke easy. Anyway, don’t you want it?” Dean spun the little vial between his fingers and Castiel watched the way the light shaped itself, searching desperately for a single point of recognition.
“I don’t expect you will just give it to me… name your price.”
Dean just glared at him, and then uncorked the vial. “Maybe I will.” The body of pure light lifted itself off the glass bottom, shyly rising into the air, climbing over the edge, twisting itself around Dean’s fingers. Castiel’s breath caught and he saw similar surprise reflected in Dean's face.
Grace spilled out, intertwining itself between Dean’s fingers, nipping at the tips, playful like a child. “Huh,” Dean said, more to himself than Castiel. He poked and prodded and the Grace splayed itself before him, letting him touch anywhere he wanted. Castiel watched it, breathless, the faint ghost sensation of imagined contact washing over his skin - he couldn’t feel it, but there was no doubt. No other angel’s Grace would be this acquiescent in the hand of a demon. He wished it was connected to him, that he could feel it.
The Grace sparked where it touched Dean's demonic form - not powerful enough to burn a Knight of Hell but sizzling - Dean must have felt it, but he gave no outward reaction.
He gestured to Castiel to come closer. “Come take it,” he sounded breathless.
Castiel gave up on dignity and crawled forward, knees and hands sinking into the soft mattress, until he was kneeling right in front of Dean. Dean grasped his jaw softly, the tenderness of it unexpected - Castiel felt tears sting in his eyes, despite his best efforts. He opened his mouth obediently and Dean leaned in, focused, the tip of his tongue pressing against his upper lip. He was cupping the Grace in his hand, where it pooled shimmering and ethereal, and then he tipped his hand lightly over Castiel's mouth. His Grace detached itself from Dean reluctantly, dripping down his fingers - Dean brought his hand lower, pressing the tips of his fingers into Castiel's waiting tongue.
Castiel closed his eyes - he couldn't look anymore, couldn't stomach the parody of tender care the demon was performing. Something real Dean would never do.
The Grace pooled in his mouth for a long moment, burning like deep freeze, and then sprung awake in recognition, rushing down his throat, making him choke.
When he possessed Jimmy for the second and final time, he absorbed his memories of their separation. In them, Jimmy returned home only to find it altered beyond recognition - or maybe it was Jimmy himself who was altered. Either way, there was no possibility of recapturing some previous state of innocence.
Similarly, his Grace felt unfamiliar as it curled in his stomach, springing tendrils out to fill his vessel, his body, from the tips of his ears to the soles of his feet. And Castiel - who had found brand new ways of desecrating himself, in the time they were apart - saw that there was no satisfaction in his return to himself.
He gasped, and the flash of Grace settling itself forced his eyes open.
He was still in the same motel room, Dean sitting close, his right knee knocking into Castiel, hand over his crotch - squeezing his erect penis. Castiel was almost grateful the demon decided to drag down the moment to the basest possible level.
“Why did you just give it to me?” he asked.
Dean shrugged. “More fun this way, isn't it?” He went for his belt.
Castiel resigned himself to the proceedings. He let Dean push him down on his back, wedge a pillow under his ass, ruck his bathrobe up to his hips - his body no longer sensitive and hurting. Castiel wished he could have the pain back.
Dean spat in his palm and massaged Castiel’s asshole, then stuck his fingers in with the barest of lubrication. Castiel gasped, pushing into the sensation, into the burn as Dean started fingering him, loosening him roughly. Ugly satisfied grin sprung up on Dean’s face, when Castiel couldn’t curb his reaction and gasped and moaned as Dean stretched him out.
Castiel didn’t like giving him the satisfaction. “I'll get him back,” he said. There was no need to specify whom he was talking about.
Dean raised his brows. “Oh yeah?” Castiel’s words barely gave him a pause and he spit in his palm again, wetting his cock in a tight grip, then guiding it in - inside Castiel. Too fast, too rough. Painful - making Castiel gasp and arch his back, grasp for the bedding, fight to keep his voice behind his teeth.
Dean slid in, settled deep with a sigh.
“You know he won't fuck you like this,” Dean said casually, pulling out and pushing back in, the slap of his balls against Castiel’s ass shockingly loud. “He won’t fuck you at all, actually.”
Castiel grit his teeth, the anger enhancing his pleasure. “I am ah,” a particularly hard thrust punched the air out of him. He lost himself for a moment, watching Dean mouth open, eyes closed, thrusting into him - pleasure came so easily to him. Well, not him. The demon. “I am at peace with that.”
Dean opened his eyes, a grin splitting his face open. “Uh-huh. Well he’s not.” He slowed down, drawing out his thrusts, and grabbed Castiel’s penis. “You better pray he doesn’t remember this.” He squeezed it, ran his thumb over the glans and Castiel moaned, helpless against Dean’s familiarity with Castiel’s own body.
“And what about Sam?” Dean asked, faux casual. Castiel sobbed and squeezed his eyes shut.
Dean increased his pace, excited by the reaction he got out of Castiel. “If you, unh, bring him back now. Unh, he’s just gonna kill himself again when he finds out what he did.” Castiel fisted the sheets, gritted his teeth. He was hurtling over the cliff, sickened by his own pathetic failure - despite everything he could not deny Dean anything - not even this twisted parody of him.
Dean groaned above him, his thrusts becoming erratic - fucking Castiel through his own orgasm. He squeezed Castiel’s penis harder and brought him off quickly, clearly losing interest in drawing this out. Castiel spilled over Dean’s hand, over his own stomach, crashing quickly. Pleasure forgotten almost immediately.
“I’ll fix it,” Castiel said, voice thin. He could fix it - he just needed to reopen the Gates of Heaven so he could get Sam’s soul back and resurrect him.
Dean patted his cheek “Sure you will.” He hissed, pulling out of Castiel and tucking his soft, slick penis back into his underwear. “Till next time, hm?” He winked at Castiel and vanished.
Castiel stared at the ceiling, no longer actively dying, but at a loss as how to keep on living.