A/N: For castielwinchestercas. The prompt was for Dean to get possessed by the tentacle monster from 13x17 "The Thing," but don't you love that I managed to turn it into a Cas whump fic? XD

Disclaimer: Opening lines from the episode; they're not mine. Thanks to 29Pieces for beta reading!

"The Thing"

Dean slowly came to and opened his eyes. Tracking his surroundings, he realized he was in the capitulum altar room…and he was chained to the altar. Wasn't that frickin' fantastic. He yanked against the bonds but they were fastened securely. He caught sight of Sandy moving around, putting ingredients into a chalice.

"Sandy, what the hell?"

"Sandy's dead. She's been dead for a long time."

Dean dropped his head back against the stone. Awesome. So the person they thought they were supposed to be protecting was actually the bad guy. Figured.

"Okay, I'll play," he said. "What's your name?"

"Yokoth. Star of Madness. Ravager of Galaxies. Mother of Faceless Hordes," she replied.

Dean cocked a brow. "Cool. Let me guess, you're not from around here."

She flashed him a manic smile as she went over to the wall and opened a hidden compartment. She pulled out a metal box. "I'm really not. When foolish humans opened the door, they had no idea what was waiting for them on the other side."

"You mean you?"

"I mean us. It was supposed to be us. Glythur and I, together. But my love…they shut the rift before he could make it through."

"Oh my god," Dean responded exaggeratedly. "That's tragic. It's like a Hallmark movie. But with tentacles."

She shot a leer at him. "I like you, Dean. You're strong. And I enjoy looking at your face."

"Whew, you're making me blush."

She grabbed his face. "That's why I'm going to allow you to be Glythur's host."

Dean stiffened. "Sorry, what? Host?"

Yokoth opened the metal box and pulled out a vial of blood and a vial of glowing light that looked unnervingly like archangel grace. "We've consumed most of our universe," she said as she added them to the chalice. "But this one is so full of…light. And of life. And it looks…" A tentacle slurped out of her mouth and licked her lips. "It looks delicious."

Dean started to squirm. "Okay, I don't know what kind of kinky Gatekeeper-Keymaster thing you got going on here, but…I think I'll pass."

She pressed a finger to his mouth, and Dean hiccoughed as something pressed down on his tongue. He swallowed and tried to gag, but nothing moved. Yokoth pulled out the Seal of Solomon and started to chant.

"Insanidox koth munto, Glythur."

The Seal began to glow purple and light shot outward and up, splitting the ceiling into a glowing rift. Yokoth slipped the chain around her neck and continued her chant.

"Insanidox koth munto, Glythur!"

Dean's eyes widened as several tentacles came flapping out through the rift. One shot toward him and he jerked to the side to avoid getting struck. The thing jerked back and lunged again, and this time Dean couldn't avoid it. It slammed into his mouth and began to push. He didn't have the breath to scream as something sick and slimy forced its way down his throat.

o.0.o

Sam bust down the door to the altar room in time to see a huge tentacle shooting down through a glowing rift in the air. The blubbery appendage slapped a four-pronged orifice over Dean's mouth, and his whole body arched as a muffled scream was drowned out by a screech.

"No!" Sam surged forward, Ophelia and Marco right behind him, but an explosion of light had them staggering back and shielding their eyes.

When it receded, Sam blinked furiously to clear his vision. The tentacle monster was gone, and Sandy was leaning over Dean, caressing his cheek.

"Hello, my love," she crooned.

Dean turned his head…his eyes glowing with eerie purple light.

"No," Sam sputtered again.

Sandy whipped her head around, finally noticing them. With a snarl, she snatched up a key to the manacles and unlocked them. Dean sat up with an alien fluidity.

Sam watched in mounting horror. "Dean…"

His brother slid off the altar and grabbed Sandy by the arms. "Yokoth, it has been too long," he said, voice deeper.

Sandy wriggled closer to him. "We are together again," she purred. "In a world full of satiation for the taking."

Dean—or the god that had taken over his body—looked around, then tipped his head back to inhale deeply. "What richness I smell," he intoned.

Sandy giggled. "And it's all ours."

"No," Ophelia declared.

Sam twisted to see her and Marco blocking the door, standing tall though there was fear in their eyes.

"We will not let you ravage this world."

Sandy chuckled. "Oh, sweetie, you can't stop us."

She charged forward, and Sam drew his gun and fired, three times, each hitting center mass. But though Sandy's body jerked from the impacts, she didn't go down. A vicious snarl tore from her lips.

"You can't kill me!"

Sam's aim wavered with indecision, realizing she was right. But then Dean was pushing past her, eyes stormy as he marched straight for Sam. Sam backpedaled. "Dean, no!"

"I am Glythur," the being boomed, and opened its mouth wide.

Sam's eyes widened in horror as a tentacle shot out toward him.

Marco shouted something in Latin, and a bright light pushed the god monster back a step. Sam dove out of the way. Another tentacle arm slapped at him, knocking him off balance and throwing him across the floor. He scrambled to his feet and saw Sandy and Dean advancing, tentacle tongues slapping back and forth.

Ophelia grabbed at his arm. "Run!"

Sam wrenched around and bolted after her and Marco. Inhuman shrieks sounded behind them as they barreled down the dark corridor and into another room where Marco slammed the door and started to barricade it. Something juddered against the wood and Sam threw himself against the door to help hold it.

He and Marco stayed pressed against it for several long minutes, but there were no more attempts to break in.

"Think it's a trick?" Sam whispered.

Ophelia's eyes were wide but she shook her head. "They're free."

And that said it all.

Sam closed his eyes against a wave of crushing hopelessness. Dean.

With a silent nod to Marco, they moved away from the door and pushed the furniture aside, then crept out into the hall. It was quiet. Sam kept a hold of his gun, for all the good it did him, as they carefully made their way back to the altar room. It was empty, though the rift was still glowing in the air.

He turned back to Ophelia and Marco. "How do we save my brother?"

The siblings exchanged a look. "We can't," Ophelia said. "They're going to devour the world."

Sam shook his head. "Not if we can get them back here. We can get them out of the people they're possessing and back through the rift to their own dimension."

"We have no idea if an exorcism is even possible," Marco put in. "And the rift will only remain open for twenty-four hours."

Sam's breath caught in his throat. Okay, he'd faced worse odds. He scrabbled to fish his phone out of his pocket.

Ophelia quirked a brow at him. "Who are you calling?"

"Backup."

o.0.o

It took four hours for Cas to meet them in Portsmouth, Rhode Island, leaving twenty hours for them to track down Sandy and Dean and get them back to the rift. Tracking them wasn't going to be the problem though. Sam scrolled through the incoming reports of mass deaths found at a gas station, roadside diner, and truck stop. The two gods were slaughtering their way up the highway, leaving a trail of shriveled bodies in their wake like an alien version of Bonnie and Clyde.

Cas pulled his vehicle up alongside the Impala outside the capitulum and parked, then got out. "Sam, any idea where Dean is?" the angel immediately asked.

Sam swallowed. "Unfortunately, yeah. This is Marco and Ophelia, the legacies who were guarding the god from the other dimension and keeping it imprisoned. Uh, this is Cas, the angel I told you about."

Cas nodded to the two siblings then turned back to Sam. "I had Rowena talk me through a transportation spell." He reached into his trench coat and pulled out two hex bags. "We put one in the room of the rift you mentioned as the destination point, and then get close enough to the beings together to transport them back."

Sam nodded. "Okay, great."

Marco took the first hex bag and went to put it in the altar room.

"Anything on how to get that thing out of Dean?" Sam asked.

Cas's jaw visibly tightened. "Rowena couldn't help with that one."

Sam swallowed. "Okay, well, we'll throw Sandy through the rift at least, and then bind Dean if we have to until we can find a way to free him."

"This is our one shot at sending those monsters back," Ophelia interjected. "We can't risk letting one stay."

"I'm not banishing my brother," Sam retorted.

"We'll think of something," Cas said in a tone that closed the matter for now.

Sam knew Cas wouldn't dream of giving up on Dean, and he hoped Ophelia and Marco had enough wariness not to challenge an angel.

Marco returned and they all climbed into the Impala. It seemed like the otherworldly beings were traveling on foot, so hopefully it wouldn't take long to catch up to them.

It took an hour and a half, and Sam was tautly aware of the time clock ticking down. But Rowena's transportation spell would save them having to drive back to Portsmouth, and once they got Dean back, Sam would be happy to have his brother ream him out for leaving his Baby out at some rest stop along the highway.

Sam slammed the brakes as they careened into the gravel lot outside the roadside diner—another thing Dean would be having a conniption over. But he could see bodies sprawled on the ground and figures running around inside.

The four of them scrambled out of the Impala and bolted for the diner, the door jingling their entrance and drawing the gazes of their targets. The tentacles that had been feasting on two customers slithered back into their hosts' mouths.

"You just don't give up, do you?" Sandy said. "But there's nothing you have that can stop us. We're gods."

Sam cocked his head. "We have an angel."

Cas surged forward, hex bag in hand. Sam and the Avila siblings leaped after him. They all had to be within range to be transported together.

The tentacles shot out from the mouths again. Cas ducked under one and threw a punch at Sandy that knocked her to the floor, leaving her stunned. Glythur snarled and his tentacle flailed around, but Cas pivoted, his angel blade dropping into his hand. In one fluid motion, he sliced at the appendage. Glythur shrieked and staggered away.

Cas held up the hex bag and shouted, "Now!"

Sam braced himself. But just as the magic began to ignite, Sandy and Glythur lashed their tentacles in a wide arc, catching Sam and the Avilas and knocking them away. Sam slid across the floor into a booth. There was a bright flash of light, and when he pushed himself onto his elbows, he gaped in horror at the empty spot where Cas and the deities had been a moment before. A gasp drew his gaze to Ophelia and Marco, also getting up off the floor.

Sam's heart dropped into the pit of his stomach. The spell had worked, but they'd been left behind.

o.0.o

Castiel stumbled as he suddenly found himself deposited in a dingy room lit with an ethereal purple glow. The transportation spell was nothing like flying with wings, and he was momentarily disoriented.

A hand grabbed his throat and swung him around, slamming his back into a wall hard enough to crack the plaster. Castiel clawed at the unyielding arm and gasped as he found Dean's fuming face inches from his own. Only, it wasn't Dean. Cold, malevolent hunger blazed from eyes devoid of the spark of that human soul Castiel knew better than anyone.

Still, he had to believe the Winchester was still in there.

"Dean," he grunted.

The hunter's face cracked into a minacious grin. "You mean my host?" He paused to rove his gaze down at himself. "My mate did choose well."

"Dean, if you…can…hear me…"

The fingers around his neck tightened.

"Angel they called you," the deity mused. "A being of light." He closed his eyes and tipped his head back with a deep inhalation. "Oh, how this will be a feast."

The being possessing Dean swung Castiel around and threw him to the floor. Before he could get up, Dean's mouth opened wide and that tentacle shot out, slamming into Castiel's chest and shoving him flat on his back. Tiny teeth pierced his vessel like a ring of barbed wire and Castiel arched his back with a scream as the entity sucked, rending life energy and grace from bone and sinew. He could feel blood coursing through his veins, throbbing against his skin as it was siphoned up as well. Every nerve ending was firing, his muscles convulsing. His cells were shriveling up on the molecular level.

And then Glythur stumbled and made a choking sound. The teeth disengaged and Castiel sucked in a ragged gasp, his lungs deflated and unable to carry any oxygen. His chest burned and his grace flared with a desperate need to survive.

Yokoth crawled over to him, seemingly oblivious to the way her mate was clutching his throat. "My turn," she hissed, and opened her mouth.

Through sparking vision, Castiel saw a glowing stone around her neck, one whose hue matched the simmering portal just behind her. Mustering his strength, he rolled sideways and lashed out to grab it, yanking the chain from her neck.

"Panto koth munto!" he barely managed to rasp, grateful the guardians of this Men of Letters chapter had gone over the spell for the rift in the car.

The tear in space and time sputtered and spritzed. Yokoth whipped her head around at the disturbance and Castiel took that moment to shove his palm toward her with enough force that she went flying backward—right into the portal's vortex. She screamed as she was sucked into it.

"No!" Glythur took off after her.

Castiel's heart seized and he flung himself forward to grab his leg, tripping him. The deity twisted with a snarl but Castiel refused to let go. His vision was going white around the edges, but he nevertheless summoned up his grace in preparation to pour the smiting might of divine glory into this invader.

It was a bluff; he would never hurt Dean. But Glythur didn't know that. With an enraged bellow, Dean's head was thrown back and the tentacle monster shot out toward the portal, abandoning his host. Dean collapsed as the deity vacated his body, and the god disappeared through the rift a moment before it closed.

Castiel clawed at Dean's leg, trying to drag himself higher to check on the Winchester, but the last of his strength fled and he fell into darkness.

o.0.o

Dean groaned as he woke to a thumping in his head and churning in his stomach. Sounds were distorted, and he could barely make out the urgent noises above him.

"Dean! No, no, no. Dean! Cas!"

Dean felt the muscles in his face scrunch up. God, this was the worst hangover ever.

Hands grabbed at his shoulders and shook him roughly. A moan escaped past his slightly parted lips.

"Dean!"

Some awareness trickled in and he recognized his brother's terrified voice. That spurred some of the stupor from his mind and he forced his eyes open. He had to blink rapidly to clear his fuzzy vision, and then Sam was there, looming over his face. He groaned again.

"Are we sure that's your brother?" another voice said.

The hands on his arms clenched. "I- I don't know. Cas? How is he?"

"He shouldn't be alive," a woman replied.

Horror jolted through Dean like a bolt of lightning and his eyes shot open. "Sam!" He'd meant to shout, but it came out more of a croak.

"Dean? Is that you?" Sam asked desperately.

"Yes, it's me," he grunted, squeezing his eyes closed again as he fought to make sense of his muddled brain. It came back with the force of a sledgehammer and he twisted away to retch over the floor.

"Whoa, easy."

"Oh my god," he moaned. "She- thing- tentacle…" He gagged again.

"Yeah," Sam said, voice heavy with sympathy. "You're okay though. It's gone. They're both gone."

"Cas," he choked out, trying to sit up and look around. He remembered now, remembered the angel's scream, felt that surge of grace coursing through his body. It resonated somehow, woke Dean from the haze of possession enough for him to realize Cas was in trouble—and that something was in him. He'd thrashed then, distracting Glythur or whoever, but it hadn't lasted long. And then…he didn't remember.

His gaze finally landed on the angel, sprawled on the floor and looking…dead. His skin was gray and almost shrunken looking, like it was pulled too tightly around knobby joints. The veins were almost protruding from the surface, a dark rust color that suggested a heart that had stopped beating.

Dean's heart lurched. "No." He crawled forward, grasping at Cas's sleeve.

Ophelia scooted away. "He's breathing," she offered, then hesitated. "But if the gods fed on him…"

"He'll be fine," Sam said staunchly, echoing the desperate plea in Dean's mind. He moved to Cas's feet and reached out to squeeze the angel's leg. "He just needs to recharge. He always does." And at that, Sam's voice cracked.

Dean swallowed the vile taste in his throat. "We need to get him out of here."

"There's rooms," Marco put in. "They haven't been used in decades. We can go clean one out real quick."

Sam gave a grateful nod, and the Avila siblings quickly excused themselves.

Dean lifted a shaky hand to Cas's throat, his gag reflex stirring at the feel of dry, leathery skin. But he steeled himself and held his fingers under the jaw line, holding his breath as he waited for that blessed beat. It was faint and slow, but there.

"What is he even doing here?" Dean asked hoarsely.

"I called him when you…went missing," Sam replied. He shook his head and rocked back on his butt. "The transportation spell was supposed to take all of us."

"He did it though, right?" Dean cleared the crackle in his throat. "I mean, I'm back."

Sam was silent for a moment, then reached forward to take a gemstone necklace from Cas's fist. "Yeah, he did."

"Is that?"

Sam nodded. "The Seal of Solomon."

Dean closed his eyes, a wave of relief making his limbs weak. They'd found the Seal. But at a cost. It always came with a cost.

The Avila pair returned and Dean and Sam hefted Cas up and carried him to the room they'd cleaned out. It still smelled musty but at least a cloud of dust didn't poof up when they laid Cas on the bed. Sam retreated back to the hall and Dean could hear him telling Ophelia and Marco about how they'd come for the Seal and why.

Dean sank onto the edge of the bed and stared at Cas's wan face. He wished he knew how to help his best friend. Any human would be dead, and that was the plus of having an angel as backup; Cas wasn't taken down easily.

But when he was, it was brutal.

Sam eventually came back, holding the Seal in one hand. He set it on the nightstand and looked at Cas. "I'll call Rowena," he said. "Maybe she can help."

Dean nodded mutely. In the silence and privacy of the room that followed, he found his thoughts drifting back to the moment when Glythur had come through the rift and…

His stomach heaved and Dean bolted to the bathroom in time to throw up in the sink. It was mostly acidic bile since he hadn't had time to eat at the diner before they'd been ambushed.

His knuckles whitened around the edges of the stainless steel sink. He wanted nothing more than to make a kinky joke about what had happened, but his body violently disagreed.

"D'n," a frail voice whispered.

Dean's head shot up and he rushed back into the room. "Cas?"

Cas's eyes were barely open, and he was making no effort to move from the bed.

Dean quickly retook his seat on the edge of the mattress and squeezed his friend's arm. "Hey, buddy, how you doin'?"

Cas was silent for several moments as his throat worked, probably too dehydrated to even swallow.

"Hang on." Dean got up and returned to the bathroom to fill a glass with water from the tap, then hurried back to help Cas drink some.

"Th-nks," he rasped.

"You're gonna be all right, right?" Dean asked, trying to phrase it as a statement that wasn't up for debate but failing.

Cas nodded minutely. "Are…you?"

"Yeah, I'm good. Monster free, thanks to you I'm guessing."

Cas closed his eyes. "Good."

Dean waited for him to say something else, but he seemed to have fallen unconscious again. Dean's heart clenched with worry.

Footsteps drew his attention to the door as Sam came back. "Rowena doesn't have a magical cure, but she gave me a recipe for a tea that can at least speed up his healing."

Dean's shoulders drooped. It was better than nothing. "He woke up for a sec. Was fine enough to care more about how I was doing."

Sam's lips twitched with the ghost of a smile, but then it sobered. "And how are you? Doing."

Dean reached up to rub the back of his neck. "I'm never gonna be able to look at sushi again."

Sam gave him a sympathetic grimace, looking as though he could read between the lines. "I'm pretty sure there's stuff here for a tea for that," he said, and then bowed out again.

Dean slumped more wearily where he sat. He didn't know whether he wanted a shower or hard whiskey. But he'd take neither until Cas was well on his way to being better. They had a battle coming up, and they all needed to be ready for it.