Author's notes: This mentions the rings I wrote about in my last fic, A Paranoid Old Man. All you need to know is they help the Doctor and Cas track each other down. Also, I left it as an open ending, so you choose for yourself what happens.
Dedicated once again to Chris. Inspired by and title taken from, "Bloodstream," by Stateless.
Castiel didn't know how it came to this—the only thing he'd been sure of was it must never happen again. Something this terrible, this wrong, it couldn't be allowed.
The moment the Doctor was struck down, Castiel felt it down to the feathers on his wings. They bristled dangerously, and his skin erupted in goose bumps as his finger was set alight by the ring—a tool, a tracking device, they moved around too much for it to mean anything—glowing gold and scorching his skin.
He hadn't realized at the time it was a warning.
He moved swiftly, without a moment's hesitation, and found himself in the bowels of an alien structure, at least two hundred relative years old. The Doctor laid on the floor, dust sweeping over his cheeks and making him appear older, sickly. He coughed and clutched at his side, groaning lowly.
Castiel's heart stopped when a different kind of dust drifted out of the Doctor's lungs—sparkling, sweet, singing with life and expelled with abandon. No matter how much Castiel willed it to be a hallucination, the image just wouldn't dissolve. It was real. The Doctor was hurt, shot low in the left lung by some kind of energy gun, and he was dying, and the blood was drying like elk fur on the floor and Castiel didn't want to see it but he did, and a spark of short-lived Huon energy landed on Castiel's nose and burnt when it disappeared, a tuft of smoke rising up and making his eyes water—
Or maybe that was just panic.
Castiel swallowed heavily—a peculiar human reflex he'd caught from the Doctor, beloved Doctor—and he knew what he had to do. The Doctor wouldn't regenerate this time around, or he would have done it already, and if there's anyone worth saving it's him. He'd heard Time Lords had this gift also, of making the ultimate sacrifice and stealing from one's own life force, even if angels used theirs to convert alien energy into usable quantities. Now's the time to use it. He knelt beside the fallen Time Lord and cupped his pallid face in his hand.
"Castiel…" The Doctor's eyes fluttered open a moment before sinking shut again, a weak smile on his lips.
"You won't like this, Doctor."
"Cassie, why's your ring burning? It hurts. Mine's burning too. Silly gadgets; th' must be malfun'tioning…"
"The rings are working well." Castiel swept his thumb over the Doctor's brow, wiping away dirt and perspiration. "They did their job."
The Doctor's face twitched in vague unpleasantness. "'re you hurt?"
Yes. "No. Doctor, you won't like this," he repeated. He had to let the Doctor know, however in a tizzy he might be. "I might not be able to see you again for a while."
"Cas?" He worked on keeping his eyes open this time, almost scowling in puzzlement. They eventually steadied, with some effort, on Castiel's face. "Cas. I know tha' look. You get it when'ver you're about t' do something stupid."
"Now is not the time for arguments." You're dying, you're fading. Huon energy was pouring out almost steadily, slipping out of the Doctor's open mouth and seeping wound, igniting little tweed threads that'd wrestled loose.
"It's always the time for… arguments…" Even his voice was fading, and Castiel knew it was time. He leant down, muttered a quick prayer, and pressed his lips to the Doctor's in an open-mouthed kiss.
Sensation washed upon him as little sparks of energy bit at the inside of his cheeks, his tongue. He could feel them crawling down his throat to rest in his lungs, surpassing the vein walls and seeping into his bloodstream. He wanted to stop; everything cried at him to stop, to relent and find another way, live another day, but he just inhaled deeper and took more in, until his alveoli were close to bursting and his fingers twitching with light and heat.
He held his breath, keeping his lips sealed to the Doctor's as his vessel acclimatized to the energy and figured out how to use its healing properties. The Huon particles danced behind his eyes and Castiel was suddenly reminded of what a miracle his Doctor was, what a beautiful thing he was risking his existence for. He didn't know if he was still powerful enough to use the Doctor's energy as his own, but if there was one thing he'd learnt it was to never give up on the ones you love. Not ever.
…not even if you could feel his blood soaking through your shirt and making the fabric stick to your skin; not even if unimaginable power was setting your body and soul alight, literally, causing the ends of your wings to smoke and curl up; and especially not even if, as you pressed your fingers to his wounds and drained yourself dry to heal him, he started to wake and murmured into your lips, No, stop, don't you dare, you stupid angel…
Castiel didn't know how it came to this, but he was going to set it right. The world could live without him; the Doctor would eventually move on. The universe wouldn't exist without the Doctor.
At least the rings had done their job.