A/N: Continuing on popular demand. Edited to fix the formatting issues.
Every time Dean wakes up, he is shocked that he is still alive. The monsters are always nipping at their heels even when they're running, and he's continually surprised that one of them hasn't gotten the drop on him and eaten his organs. He figures they must be keeping a pretty low profile, or maybe Cas is just giving them that scary look (although it never seems to work when he's awake).
When he wakes by himself, for once, it is because a spray of warm monster blood has just struck his face. He scrambles to his feet and grabs for one of the wooden stakes they've made during their time in purgatory (once it had been decided, silently, that makeshift clubs weren't going to cut it) and finds it not there. The reason for that would be his angel, who is holding both of them and slashing brutally at a hissing denizen whose former identity Dean doesn't even want to guess at.
His angel. It's been a while since Cas has been that.
He doesn't dwell on that too long, though, because his angel or not, Cas isn't going to be around much longer if he doesn't get some help. He's been limping for a few days, and while they don't bring up their respective injuries, it's slowing him down. Just like the sluggishly oozing wound on his abdomen is slowing Dean down.
He whirls and snaps a hefty branch off one of the ever-present dead trees and throws himself into the fray, clocking the thing on the back of the head. It snarls and turns, giving Cas a chance to get a good stab in. It roars, and Dean slams the chunk of wood into its face.
It staggers then, and Cas plunges one of the crudely fashioned stakes into its chest. It growls, or at least Dean's pretty sure that's what it was supposed to be, but it comes out more like a strained gurgle and the thing topples, the blood streaming from its chest and staining Castiel's hands.
They stand there, silent, catching their breath, before he speaks. "You're awake."
"Uh, yeah." Because what else can he say, really? "This happen a lot?"
The angel shrugs. "No more than can be expected."
Dean's pretty sure this is angel-speak for 'every freaking night'. "Why don't you rest one night, let me keep watch?"
"I do not require sleep."
It sure looks like you could use it, he thinks, but he doesn't voice this. Mostly because Castiel is stubborn and he's not sure he could keep the things off them all by himself for…however long he normally sleeps, anyway. He used to ask how long he'd been resting, but neither of them could keep any accurate count of anything—whether minutes or days. They stopped for sleep whenever Dean could go no longer without it, not when the sun set.
There was no sun.
Dean is pretty sure it is getting darker. He'd thought it creepy, at first, when he could see with no light source. But he's starting to miss it, because he's pretty sure the shadows are getting darker and the trees are looming more and the ground beneath his feet is shakier and he can't see more than a couple feet ahead of him.
"Hey, uh, Cas," he brings it up. "You starting to notice we can't see?"
"I'm sorry," the angel says, and Dean just stares at him.
"Dude, why are you frickin' sorry? Not like you're making it darker."
"I'm not," Cas agreed, "but I'm not making it lighter."
"Sorry?" Because he had thought Cas had recollected his marbles, but now he isn't so sure.
"I was…letting us see," Cas admits, and suddenly it makes sense to Dean, "But...I'm sorry, I'm getting too weak."
"Hey. No big deal." And Dean is a bit struck by the fact he'd just been walking around letting Cas expend his energy on this while the angel said nothing.
"I'm sorry," he repeats, and Dean turns and catches him by the shoulder.
"Hey. Not like I was doing anything, was I? Stop being sorry." They stare at each other for a second, a minute, before they turn and keep walking.
The silence is making Dean's skin itch. "We'll figure something out," he assures Castiel, even though he has no idea what. He gets the idea when they are alerted of the next encroaching monster by the gleaming eyes.
To his credit, Castiel finds a suitable stick to impale the eyes on when Dean cuts the first pair out.
The next sign Dean gets that his angel is fading is when they stop teleporting. When he wakes up, Cas hauls him to his feet instead. "We need to run," he says, seriously, and Dean knows that he means 'run' quite literally.
"You okay?" he asks, when they've stopped and he's got his breath back but Cas hasn't.
"I am fine," he insists, but makes no effort to straighten. Maybe he knows Dean wouldn't believe it anyway.
"Dammit, Cas," he snaps, and the angel visibly flinches and Dean relents slightly. "You gotta take a break."
"When I am dead," he responds, and Dean isn't sure if he's being serious or if it's a poor attempt at humor. Either way, it effectively ends their conversation, but when they stop to rest again, Dean suggests that Cas sit down.
"I'll keep an eye on things," he promises, and Castiel slumps down against a tree. He doesn't sleep, but his eyes are glassy and he's so out of it he might as well be. Dean holds a stake in one hand and their skewer of glowing red eyes in the other and waits for the other shoe to drop.
He dispatches a violent rat-creature after what feels like a few hours, and becomes aware of a dull ache in his side. Upon examination, he notes the wound in his side has split open again and is oozing blood. The skin about it is red and inflamed, and when he moves too suddenly, a jab of pain shoots up his side.
"Well, damn."
Another indefinite period of time later, he has traded the wound on his side (barely healed) for a series of slashes on his shoulder and a couple of broken ribs. For his part, Cas is still limping, and Dean is beginning to worry that whatever damage was dealt to his leg isn't going to go away by itself.
That's hardly their worst problem at the moment, though, because if he could compare this thing to any species known to man, he would say dragon. Only it's not exactly breathing flame so much as gushing it from its eye sockets and Dean is almost happy to see some form of light that isn't gleaming red or draining Castiel's life away until the thing attacks them, slashing with razor sharp talons and snarling with hooked yellow fangs.
"What—is—this?" He gets out between struggling for breath and dodging a swipe of its spiked tail.
"Tiger?" Cas suggests, deadpan, and Dean almost grins.
"I wish."
He has to admit, the black sludge and flames it explodes into when they finally finish it off are pretty impressive, even when scrambling to light up a torch (it might not last forever, but it would be handy while it lasted). The corpse burned quickly, and Dean had the feeling that the icy, eternal night quenched fire faster than it should.
"So, tiger, huh?" he asks, as they crouch by the dying embers and try to enjoy the warmth while they can.
"In what distant deeps or skies, burnt the fire of thine eyes?" Castiel quoted, eyes locked on the struggling flames that look like they're having the life choked out of them.
"Huh." He sits there for a minute, absorbing it. "What's the next bit?"
"On what wings dare he aspire? What the hands dare seize the fire?" He speaks quietly.
Dean feels the need to continue, but doesn't know the rest, so he repeats the lines that have stuck in his head from before. "Tiger, tiger, burning bright, in the forests of the night."
Their eyes lock on each other as the last of the flames fades to nothing.