In the manner of most important events, it happened suddenly.
Later Castiel would recall the moment in a series of facts, a sequence of images tucked carefully away in the back of his mind, to remember: a clutch of demons, the wide empty night and a confrontation somewhere in the depths of the woods, the whirling of trees. He would picture the blur of the fight, the colour and the noise. Gunshots, arcs of dark blood and a gibbering, a keening—and he would recall the thought that the noise Hellspawn made as they died was like the yelping of kicked dogs.
Almost precisely five months after the End had not come, nearly half a year after Castiel had turned his back on Heaven for good, he had a palm pressed to a demon's temple and its life was seeping out through its eyes when the last tendrils of his grace finally snapped.
He had screamed, in that instant. He had stumbled away from the demon's wilting body and landed hard in the grass, on his knees, feeling something huge and horrible flickering and swelling in his chest, like a white fire—he had lost track of sound and the battle and without knowing what he was doing he had felt himself turn inside out, had thrust fingernails into the searing, howling pain, and ripped.
It was only after he opened his eyes to silence and the wheeling night sky overhead, after he felt two pairs of hands grasping his arms and lifting him up, only after he felt his vessel's heart beating in a chest that felt ten times too hollow, that he realised. It had happened.
He had Fallen.
Sam and Dean had slung their arms around his shoulders—for his support as much as their own, for in his daze Castiel had seen that they were bloodied and bruised, that Dean had claw-mark gashes down his cheek and Sam had a blown pupil—and Dean had clapped a hand against his back and said, “You okay? The hell was that?”
And Cas would remember, vividly, that he had tried to speak.
He had tried to say, “I'm alright. My grace broke, it snapped—I tried to pull it out, and I think I've Fallen, now, and it didn't hurt as much as I thought it would.” But though the words pushed forward onto his tongue there was no voice behind them—only air—and he stopped short.
The brothers paused and looked at him.
He would remember that night as being chill, and feeling goosebumps rise on his bare arms. He had a gun tucked into the waistband of his jeans, against his back, and he was wearing one of Dean's old T-shirts, a pair of Sam's old boots; he had smitten two demons in the melee and their bodies lay smouldering on the grass behind them; he had the faintest sense that something, the way they had just come, was growing, something that smelled like new wood and furling leaves, like cold arctic air and paradise fire. And he would recall, for weeks and months afterward, the sheer shock of speaking and hearing no voice come out.
He had tried, again, to say something, but no matter how he moved his lips or pressed at his throat with pale fingertips no sound emerged.
He had almost panicked, there, for a moment—panicked because silence was such a foreign thing, to hear on his own tongue, to feel in his own throat, he a creature born to sing and praise, all speech torn off in an instant. He had gesticulated towards his throat, eyes wide, and Dean had seemed to understand.
“Hey—it's okay, don't panic, alright?” he had said. Collected, practical, as always. “Let's get out of here and then we'll see what's up.”
They had climbed into the Impala, and Cas had leaned against the window, swallowing reflexively as if trying to massage voice back into himself.
Over the tops of the trees he had seen it—branches, flowering and climbing, bursting through the leaves and reaching towards the sky, rich dark wood curling and knotting and pushing. His grace gone to seed amongst the bodies of demons.
Part of him had found it beautiful.