At some point in the fight the creatures surround Cas. He had explained to Dean that while a human soul could draw unwanted attention, what will probably drive them crazy was going to be his grace.
"Dean, run!" he screams as he trusts his angel blade through one of the creature's eyes. Dean keeps hacking with the demon knife, trying to get back to Cas, he knows his chances of surviving without Cas would probably go down to negative anyways.
"I'm not leaving you here Cas"
"Dean I'll find you again. Go!"
Dean's arms are growing tired, his steps begin to feel uncertain; the forest floor turns muddy with all the blood and guts left by the creatures that keep getting up and attacking them.
"You said it." the knife gets caught in a creature's neck, "We don't split up." Dean kicks another one that is lying on the floor, willing it to stay down. "We won't be able to find each other again."
He can hear the strain that this is taking from Cas too.
In a moment, Cas is on the other side of the clearing with too many creatures surrounding him, the next he's at an arm's length from Dean, placing a fist of something bloody in his free hand, the creatures still clinging to him through teeth and claws.
"Take this. I will be able to find you as long as you have it." Cas' eyes and the blood spilling from his forehead are the only colors Dean can see. "Now GO!" he says as Dean feels an invisible force pushing him outside the clearing. The creatures don't seem to notice that he's gone, all too happy to keep gnawing at the angel till they find the tasty gooey center full of grace.
Dean runs before he can think better of what he's doing, willing his aching feet to keep running, to keep working after all the time he had of not stopping.
It is not until he finds in a grove a tree trunk dry enough to hide behind that he takes a look at what Cas has given him, his hand is still smeared in blood as he opens his fist, finding two bloodied feathers lying in his open palm. He's shaking, he realizes that the blood in his hands probably doesn't belong to the purgatory creatures only, but to Cas as well, and he probably should have known this by now, and he does, but knowing something in theory and finally seeing it applied to reality, to this reality, are two different things. Now he knows Cas can get hurt too. Now he knows the rules don't apply to him either. Dean keeps shaking for a while, crying won't help now Winchester. You gotta keep going, Dean reminds himself.
He cleans the feathers with his saliva, since you can't even trust the water in here. He scrubs them with his fingers until they turn a pink color instead of the vivid red they were and he wipes what's left of the caked blood off on his jeans. He takes a deep breath, stands up and keeps on walking.