Dean felt a dark smile on his lips as he advanced on Cas. The angel's imploring words filled the air unheeded as Dean cornered him in the dark library. Cas looked like he was going to put up a fight, and Dean's smile grew wider. Cas threw the first punch, but Dean caught his arm and twisted it until he heard something crack. With his other hand, he struck Cas across the face, sending him stumbling, but a second later, Cas was upright again, his hands on Dean's chest begging him to stop, begging him not to let the Mark of Cain control him. It was laughable, seeing the once-powerful angel bleeding at his mercy. Dean hit him again and this time Cas lost his balance and fell backwards onto the pile of books in the centre of the room, two other bodies already lying beside him. The pounding of his own adrenaline-spiked blood in his ears made him deaf to Castiel's words as he lifted him by the shirt and hit him again and again. With each blow came a cry of pain. Cas looked up at him, silently begging him to stop.

"Dean," he choked through the blood that was filling his mouth, but the angel made no further sound as Dean lifted the angel blade from Cas's sleeve and drove it into his chest. With a flash of blue-white light, his body went limp, and Dean dropped him to the ground. He looked at the empty vessel with satisfaction, and remained watching as the angel's bright blue eyes dulled and glazed.

Dean was woken by a sharp, painful gasp. His eyes snapped open and he found himself immediately sitting upright in bed. He threw off the covers, opened the door and tore through the long corridors of the bunker, pulling himself round corners until he reached the library where he found Cas sitting at one of the large desks. Seeing his expression, Cas stood and opened his mouth to speak but Dean didn't give him the opportunity; he grabbed the angel and pulled him into a tight embrace. He felt his heart hammering against his chest, and his shuddering breaths that seemed magnified against Cas's body. Just holding Cas started to steady him, and after a few seconds, he stepped back.

"What's wrong?" asked Cas, his brow furrowed, his eyes concerned. Dean realised before he had even spoken how feeble the words would sound, but nevertheless, he voiced them.

"I had a nightmare."

"What happened?"

"It was back when I still had the Mark of Cain… we were fighting in here, like before, but this time I didn't stop and I…" he couldn't finish the sentence.

"You killed me," Cas finished, his gravelly voice sombre. Dean felt a cold wave course through him at the words, but he nodded slowly. "But you didn't Dean. I'm still here, and the Mark is gone."

"I know. It just felt so real. I could feel everything, and I…" he choked on the words. "I enjoyed it."

"Dean," Cas said, more firmly this time. "That was never you. That was the Mark influencing you. You know you're a good man, and you know you'd never hurt me, never of your own volition."

"I know," he said quietly.

"It's late," Cas said, glancing at the clock over Dean's shoulder. "You should rest."

"Yeah, okay," Dean conceded, turning a little reluctantly to the door, but not before he took one last look at the angel, committing him to memory as he stood there, very much alive.

The rest of the night passed in a broken but mercifully dreamless sleep. The following night however, the nightmare paid him another visit. It was almost exactly the same as the night before, except this time, as Cas lay at his mercy, pleading for his life, Dean discarded the angel blade, and he decided that he would get much more satisfaction from beating the angel to death with his own hands. Cas fought feebly against Dean as his face grew bloodier and bloodier, his armed pinned to the ground by Dean's knees. At long last he stopped fighting and fell limp.

Dean was awoken again as the sick pleasure melted into horror at what he had done. Unable to help himself he stood and went immediately to find Cas. Amid all the terror and guilt, the only other thing he felt was a desperate need to see the angel's face. This time he stopped in the library doorway, resting his head against the wood as he caught sight of Cas, reborn, safe. Cas looked up, and almost immediately a look of sympathetic recognition came across his face.

"It happened again," Cas said. Dean nodded. "Is there anything I can do?"

"I don't think so," Dean said. "Once I see you, I'm fine. Even when I wake up and I know it was a dream, it still feels like it might be real until I see you."

"Well I'm not going anywhere," Cas said, with a reassuring smile.

A week passed, and every night, Dean was hounded by the same nightmare, but every night it became more graphic, more violent, more real. He was exhausted. Dark streaks had formed under his eyes, and his cheeks looked sunken and sallow. Sam and Cas had started watching him carefully, as if he might keel over at any moment, but besides the exhaustion, Dean felt relatively normal during the day. He continued to work cases, watch TV, go to bars, but as the afternoons drew in and the sky grew dark, a mounting feeling of dread swelled inside him. His bedroom became a place of fear instead of safety, and when he stepped inside in the small hours of the morning, closed the door and switched off the light, he felt the dreams begin to circle him in the dark, closing in.

About two weeks after the dreams began, Dean opened his eyes to see the red glow of his clock that told him it was five past eight in the morning. It was the first night he hadn't been tormented by the nightmare. Before he could begin to wonder what had changed, he became aware of a weight on the other side of the bed, and the distinct sound of someone else's breathing. He rolled over to see Cas sitting on his bed, his back straight against the headboard.

"Uh, Cas…" Dean said, his voice a little hoarse. "What are you doing?"

"An experiment," he said, simply. "Did you have the nightmare?"

"No."

"Good," he smiled, getting up and heading for the door.

"Did you sit there all night?" Dean asked.

"Yes."

"Thank you," he said, slightly bewildered, but nevertheless relieved to have had a full, uninterrupted night's sleep. However, as the day went on, his gratitude faded to be replaced with slight embarrassment.

"Would you like me to sit with you again?" asked Cas, as Dean prepared to go to bed.

"No, it's fine," Dean said, hurriedly. "I'm sure I'll be okay." He would have felt vulnerable asking the angel into his bedroom to protect him from something that wasn't even real, so his own inhibitions propelled him along the corridor to his small, windowless bedroom alone.

It was only a matter of minutes before realised that he had made a mistake, as the familiar fear of sleep came over him, that tightening anxiety in his chest as he imagined how he might murder his best friend that night. But he knew he couldn't predict it. His actions in the dreams were always far beyond anything he could imagine in brutality and violence; he would just have to wait for the inconceivable horrors that, in a few short hours, would be seared into his mind until the day he died. Before he knew what he was doing, he was back in the library.

"Would you stay with me?" he asked. Cas gave a small smile and nodded.

"Of course."

It became their habit, and Dean quickly stopped feeling embarrassed and began to look forward to sleep again. Two weeks later, however, Dean awoke to find his head resting on Cas's chest, the angel's arm hanging casually over his shoulder, while he held a book in his free hand.

"Sorry," Dean mumbled, pushing himself off hastily.

"Don't be," Cas said.

That night, Dean closed his laptop and headed for his room, Cas following behind. But he was suddenly reminded of the awkward position in which he had woken up that morning, and, more concerning than that, how very comfortable it had felt.

"Uh, you know what," Dean said, stopping and turning to face Cas. "I think I should be okay tonight. I think I've got the nightmare thing under control." For a moment, Dean thought Cas looked a little crestfallen, but a second later he had a smile on his face.

"I'm glad," said Cas. "Goodnight, Dean."

The nightmare woke him again early the next morning, but instead of horror he felt a wave of relief, because this time he hadn't killed Cas. He had managed to see through the red haze that invaded his mind, and as he looked down at Cas, pleading once again for his life, he could hear his own voice in his head, telling him that he had to stop, that he didn't want to hurt the angel. Now that he was awake, he still felt guilty for hurting Cas at all, but it was progress at least.

Deciding he would go and tell Cas about his relative success, Dean tried to sit up but quickly realised that he couldn't move. He felt a sudden rush of panic to find that his limbs were pinned to the bed; he was completely immobile except for his eyes, and he knew that he hadn't really woken up at all; he was still trapped inside his nightmare. He heard a drip land on his bedclothes and looked down to see something dark staining the duvet. Another drip, but this one landed on his forehead and it felt hot. He looked up to the ceiling where Cas was suspended, eyes wide and terrified as he looked silently down at Dean, a deep gash across his abdomen. Dean knew what was coming next before it happened. With no warning, the room was suddenly flooded with light as Cas was engulfed in flames.

"Cas!" Dean screamed for him, fighting against his invisible bonds, trying desperately to reach Cas as his screams of pain filled the air, mixing with his own. He couldn't look away, his eyes trained on Cas as the flames consumed him, all the while struggling in vain to save him.

Through the screams, Dean heard someone say his name, he felt pressure on his arms and suddenly he was awake, really awake this time. Cas was sitting on the edge of the bed with his hands on Dean's shoulders.

"Dean, it's me. Wake up!" Dean sat up swiftly, his fingers clutching tightly to Cas's forearms. His breathing was coming in ragged gasps as he tried to calm himself, reminding himself over and over that Cas was fine. He looked up at the angel who was watching him with deep concern in his eyes.

"Was it the same dream?" he asked. Dean shook his head.

"Worse," he croaked.

"Do you want to talk about it?" Dean shook his head again.

"Not tonight," he said. He finally released his grip on Cas, taking a few slow breaths. "Would you stay with me again?" he asked, all concern for his self-consciousness disregarded.

"Of course," Cas said, kicking off his shoes and climbing onto the bed. They didn't speak as Dean lay back down and Cas settled himself in a sitting position, but Dean deliberately shifted his arm so that it was touching Cas's hand. It seemed to ground him, and remind him that the angel was safe by his side. Several minutes passed before enough of the adrenaline had faded from his system for him to become tired again. As he looked up at the white ceiling, listening to Cas's soft breathing beside him, he realised that he was becoming less afraid that he might hurt Cas, and far more afraid that someone or something might take his angel away from him.

Although the nightmares had stopped altogether, Cas still sat with him while he slept, and even when Dean woke up one morning with his arm stretched across Cas's chest, he didn't suggest that they put an end to their new arrangement. In fact, as the days and weeks passed, Dean found himself edging close and closer to the angel.

The fact that they were spending the night in the same room didn't seem to concern or surprise Sam in the slightest. One night, as Sam was passing Dean's door, he saw them both going inside and gave a small, knowing smile.

"Goodnight," Sam said, mischievously.

"It's not what it looks like," Dean said, dryly, hearing the amusement in his voice.

"Okay," Sam conceded, but the little smirk didn't disappear as he walked along the corridor to his own room. Dean went inside and pulled off his jeans, folding them neatly and hanging them over the back of his desk chair.

"What did you mean when you said 'it's not what it looks like'?" asked Cas. Dean groaned inwardly and he took off his socks.

"Really?" he asked.

"I didn't understand the reference."

"Sam was suggesting that we might be doing something other than sleeping in here."

"Like what?" Cas asked, his head tilted slightly to the side as he watched Dean with curiosity.

"Like having sex, Cas," he said, his embarrassment manifesting itself as irritability.

"Oh." Dean climbed into bed and Cas assumed his regular position on top of the covers on the other side of the bed. "We could if you liked." Dean inadvertently stopped breathing.

"We... what?"

"Well we do share a bed," Cas said, shrugging casually, but Dean didn't miss the look of anticipation in his eyes.

"Yeah…" Dean said. "That's true." He propped himself up against the headboard and turned to look at Cas, studying his face carefully. "You do know what you're suggesting?" Dean checked.

"Yes," Cas said. "I may not understand certain figures of speech, but I'm not completely naïve. And I'm not completely devoid of human desires either."

"I'm beginning to notice that," Dean said, surprised to find that, of the many emotions he was experiencing at that moment, hesitancy was not one of them.

Sam closed his book and laid it on the nightstand before switching off the lamp. He took a few moments to get comfortable, but soon he could feel himself slipping into sleep. But he was suddenly upright again when he heard the sound of something smashing in Dean's room. He pulled his gun swiftly from the drawer beside his bed and crept out into the corridor, the only sound now was his pulse hammering in his ears. He readjusted his grip on the gun, edging closer and closer to his brother's door, listening intently for another sound. He pushed open Dean's door, gun raised, and saw Cas on top of Dean, kissing him, wearing nothing but his white shirt with his knees either side of Dean's hips, Dean's hands in Cas's hair. The bedside lamp lay shattered on the floor where, by the looks of things, Dean had knocked it off with his elbow.

The bedroom door hit the wall, Cas whipped around and Dean sat up to see Sam standing there. Sam was surprised to see the complete lack of embarrassment on Dean's face.

"I don't know what to say, man," said Dean with an apologetic grin. "It's exactly what it looks like."