Cas gasped a mighty breath of air, his eyes opening wide in terror, his hands flailing over his head. Something heavy was hitting him repeatedly on his left shoulder with a dull thud. His stolen blood was pumping impossibly fast, thundering through his ears, deafening him. He scrambled frantically backwards, pushing himself on his back with his legs. Every time his heel caught the hem of his coat his heart seemed to stop momentarily.

He had to get away, he had to run, fly, now, or he was dead, and there would be no one to warn Sam and Dean – and Dean was a demon and he had to help Sam cure him, or deal with the demon if the cure didn't work, he had to get to Earth, now, or everything would collapse, his world would end and his best friends would die and – and –

"CAS!"

Suddenly his shoulders were pressed firmly against a solid something, and with that abrupt contact came a greater awareness of the world around him. A world that was most definitely not Heaven.

"CAS!"

Panting, eyes wide, Cas lowered his arms and stared at the apparition in front of him. It was Sam.

"S-S-Sam?" he stammered, wondering if this was some sort of hallucination caused by extreme stress, or death. Maybe instead of their lives flashing before their eyes, angels saw the people they loved most.

But then, he wasn't an angel.

Fighting to get his heart rate and his breathing under control, Cas squinted up at Sam. He looked solid, real. Widening his gaze, Cas recognised the Bunker's atrium. Silent books watched from their shelves as he pushed himself to his feet.

"Um," he said, utterly confused. "What ... happened?"

Sam's face broke into a brief grin, and he hunkered down beside Cas, keeping his palms up in a gesture of peace. Had he thought Cas had been trying to defend himself from him? "You sure you're with me?"

Not sure at all, Cas nodded.

Sam exhaled in a relieved gust. "Good. You're in the Bunker."

"I can see that," Cas said, keeping his tone carefully level and casual. Probably too casual. "But, um ... how did I get here?"

"I summoned you."

"You summoned me."

Sam nodded, his eyebrows rising.

Cas stared at him intently. "From Heaven?" he clarified.

"If that's where you were, then yeah, from Heaven."

"Just now?"

"Yeah ...?"

Cas blinked. Well. That was very fortunate timing. Not for the first time, Cas looked down at Jimmy Novak's body, pleasantly surprised to find himself alive. He looked back up at Sam, smiling. "Thank you."

Sam returned the grin. "Anytime. Hey, Cas?"

"Yes?"

"You look like crap."

"I feel like crap," he answered truthfully. In fact, 'crap' was a massive understatement. With the initial terror and confusion of the unexpected relocation – not to mention narrowly avoiding Metatron's angelblade – Cas could feel the emptiness regaining its hold inside him. Quite suddenly, he found he couldn't hold himself up any longer.

He slumped to the side, and Sam reached an arm out to stop him. "Whoa, buddy, you alright?"

"No," Cas breathed, his voice tight with pain and fear. "I, um ... Sam, take these. Quickly."

He held out the rough sack he had taken from the office. Or rather, he held out the neck of the sack: his arm was too weak to hold its weight.

"What's this?"

"The ... the Tablets."

Sam's eyes widened as he took the bag. He glanced between it and Castiel and his brows quickly pulled into a perfect Winchester frown. "Your Grace is almost gone, isn't it?"

Cas nodded. "M-More than almost."

"Right. Come with me. I have it all ready."

Cas wanted to ask all what he had ready, but Sam was pulling him to his feet much too quickly and his breath seemed to have stayed on the floor with his stomach. Pulling Cas's arm over his shoulder, Sam half-carried him through the Bunker and into the dungeon.

A table waited to one side of the iron Devil's Trap. A large, wide bowl waited atop it, and Cas caught the faint aroma of something sweet and woody that tickled his memory.

"You wait here."

Sam deposited him on the floor outside the Trap. Cas looked around and saw red lines painted onto the concrete beneath him. He was sitting in a sigil.

Apprehension twisted inside him.

"Sam?" he called. "What are y ... What are you doing?"

"Making you a Grace," came the reply from somewhere to his left. He felt his brows pull down in a frown.

"That's impossible, Sam."

"Nope. Just tricky."

"Sam, what do you intend to do?" His words were drowned out by Sam's as the young Winchester began a long Enochian chant. Cas tried to concentrate on the words, but they kept sliding in and out of his hearing, like a badly tuned radio. He caught a few choice words though, and they did not reassure him.

"Saaam!" he called in warning, trying to pull himself to his feet and failing impressively.

Suddenly Sam was standing over him, his feet waiting just beyond the outermost painted line. Cas felt Sam's fingers draw something on his forehead with a dry paste, chanting all the while.

"Sam ..." Castiel's protest went unheard as Sam daubed more of the sweet-smelling paste on his cheeks.

In one fluid motion, Sam knelt down opposite Cas. His eyes were closed, his eyebrows creased in concentration. Too quickly for Cas to stop it, Sam thrust his arm out, slamming his palm against Cas's forehead and holding it tightly. Cas heard his angelic name ring out and echo through the dungeon as Sam's eyes opened, his irises burning a brilliant blue.

Then, as Castiel opened his mouth to scream, the world exploded.