A/N: Prompt by meupclose on tumblr: Leviathan!Cas walks towards the water and goes under. Dean watches shocked for a moment but braces himself to fight whatever may come out. The trench coat floats near him and pain fills his chest. He looks out to the horizon before a body floating face down catches his eye. He screams "CAS!", before diving in the water to save his friend.
This wasn't the end – it couldn't be the end; not his end, not Castiel's… Dean wouldn't let him go like this, not after he had just gotten him back – no.
Dean's heart was hammering so hard in his chest that it was tattooing against the inside of his ribcage. Cas was not Cas, he was not whole – not the friend, the guardian, that Dean had come to know him as. His eyes were vacant – long dark tunnels which contained something horrible in their depth. The blue irises that refracted the light through them as delicately as the stars strewn throughout the galaxies had been snuffed out, filled with black grime. Cas' internal war with the leviathans had begun, and as Dean's eyes followed the pained movements of Cas' jerking muscles, he knew the battle was being lost...
The slow, disjointed movements of Cas were leading him further and further away from Dean, towards the water. A black liquid beginning to seep out from his tear ducts, dripping down his face and leaving stains smeared down his cheeks. Dean could hear Sam's heavy breathing next to him as Castiel waded out into the edge of the pond. He paused when he was thigh deep, and Dean desperately fought the urge to charge after him and drag him out. The black gunk was dripping out of the corners of Cas' mouth now; his whole body appeared to be contorting, being overpowered by the beasts inside. Without warning, Cas plunged down – completely immersed into the water; his head disappeared under the surface, creating ripples that disturbed out the rest of the pond.
The space in between the pounding of Dean's heart felt like the world was standing still around him – his eye raked the water surface, waiting for Cas to resurface. The ripples died down and the water became calm – with no sign of Castiel. The water was suddenly marred with a shot of black that spurt from the place where Cas had disappeared and permeated through the entire pond, and then it was gone. Dean braced himself, he was ready for Cas' head breaking through the surface of the water – ready for the beasts that had been inside him to pour forth out of the water; he was ready to fight anything, as long as Cas was alright. His anticipation built, but nothing happened. There was nothing coming.
"Dean…" Sam's voice was vaguely audible from behind him, but he wasn't listening to Sam now – not when he was sure that Sam would say something he didn't want to hear; that he couldn't hear. Then Sam's hand lightly touched Dean's shoulder and he flicked his head round in annoyance to look at his younger brother; but Sam's features were soft, vulnerable. Sam was pointing, and Dean followed the line of his finger to the edge of the pond. Floating on the surface of the water, was Cas' trench coat- black stained and sodden…
"No…" Dean shook his head, an iron grip tightening in his abdomen. Cas' coat, without Cas… The tight sensation was rising up, filling his chest with what felt like ice cold liquid – compressing the air out of his lungs and closing his throat. He bent down, reaching out a hand – desperately hoping and wishing that when he touched the trench coat, Cas would be there too. But the coat was empty; the material sagged with the weight of the water as he scooped it out. He stood motionless, the trench coat draped across his arms and the water soaking out of it, dripping onto his feet. "Cas…" A lump had formed in Dean's throat, blocking his airways and making his eyes sting with the formation of tears; Cas couldn't be gone, but he was…
Dean dropped his head, so that neither Sam nor Bobby could see the tears falling from his eyes, and stared at the coat – his fingers tightening into the wet material; how could this be the only thing left of Cas? Screwing up his eyes, Dean tried to stop himself from shaking – he wanted to sob, he wanted to let this anger and tension out; but he wouldn't – not now, not while he was being watched. It felt like a very long time that Dean stood there; his feet rooted to the ground, weighed down with the situation that had just unfolded in front of him. He felt like he was in the water with Cas, the pressure bearing down on him - like he was drowning on dry land...
"Dean?" Sam spoke behind Dean, "I'm sorry Dean, we should go..." He let out a shaky breath and opened his eyes, looking out across the surface of the pond. His heart jolted in his chest as his eyes caught a dark shape - floating way out towards the horizon, nearly at the other side of the pond.
"Cas…" He whispered, his voice cracking; the trench coat dropped from his arms as, without a second's thought, he bound towards the edge of the water. "CAS!" Sam and Bobby seemed to have been startled by his sudden movement, but their calls couldn't divert him from what he was going to do – if that was Cas, he had to save him, he didn't care what else was in that body of water!
Diving headlong into the water, the coldness forced the air out of Dean's lungs and the water soaked into his clothing, pulling him down – but he swum as hard, and as fast as he could, to make it to the body. Castiel's body was floating face down in the water; Dean grasped his hand around Cas' upper arm – turning his body over. Cas' eyes were open, staring straight up and blank:
"Cas? Come on, Cas…" Dean urged Cas, shaking him gently and kicking his legs frantically to keep his head above water. He fastened his arms underneath Cas' armpits and began to kick hard, pulling Cas along towards the shoreline. "You're okay, come on Cas…" He murmured, out of breath as he fought against the water and the weight of Cas holding him back. Sam had waded out into the water to help Dean; he grabbed one of Cas' arms as Dean made it to him and helped to haul him onto the bank. Dean was panting from the effort and beginning to shiver as the cold crept into his skin, but Cas was the most important thing.
"Cas?" Sam was shaking Cas' shoulders, but there was no response; he bent over Cas' face. "Dean – he's not breathing?"
"Pulse?" Dean forced out, collapsing onto his knees next to Cas' body, Sam's fingers roved over Cas' exposed neck.
"Nothing." Sam shook his head fiercely; instantly interlocking his fingers and beginning compressions on Cas' chest. "Come on Cas, don't leave us now." Dean watched as Sam thrust down hard, trying to restart Cas' heart. "Dean – can you…?" But Dean had reacted before Sam had asked: forcing Cas' mouth open, taking a deep breath and blowing into his mouth. He repeated this and then looked at Sammy, whose fingers were back on Cas' neck, checking for a pulse.
"Anything?" Dean asked, Sam shook his head and began compressions again. "Don't do this Cas! Not now, you son of a bitch!" Dean took a deep breath as Sam paused and breathed so deeply in Cas' mouth that he wished that he could pass part of his life force through this. On his second blow of air, a low gurgling noise emitted from Cas, and suddenly his back arched and he coughed violently. Sam grabbed his shoulder and rolled him onto his side, so that the water Cas was hacking up, didn't block his airway again. Dean let out an audible sigh of relief as Cas chocked; he was alive – he hadn't lost him. Cas' blinked; he looked as though he was coming out of a deep sleep – staring up at Sam and Dean above him.
"Thank God," Dean whispered, his eyes stinging again as his heart filled with overwhelming relief – he grabbed Cas' hand and squeezed it tight. "I thought I'd lost you Cas."
"I…" Cas spoke, but his voice was weak, "I'm not… not leaving you yet."
A/N: So this turned out longer than I thought it would be, I hope you enjoyed reading it - I'd love to know what you think of it! :)