It began when Castiel opened Purgatory. He did it to stop Raphael, to end Heaven’s civil war once and for all. But when he took in every soul — and every Leviathan — something ancient stirred within him. Their hunger fused with his grace, reshaping it into something neither holy nor damned. When the Leviathans were purged from his vessel, what remained was their sickness: a parasite written in the language of creation itself. The angels would later name it Pestis Gratiae — the Plague of Grace.

At first, no one knew. Castiel came back from the depths changed, but alive. His power grew tenfold. His miracles were sharper, quicker, more absolute. When Dean was cut open in a hunt, Castiel healed him instantly — but the skin knit too tightly, leaving behind a web of blackened scars.

They told themselves it was stress. That Heaven’s energy worked differently now. That maybe grace left marks because it had to. But then came the smell — ozone and rot — and the faint black glow that followed in his shadow.

Now, deep within the Men of Letters bunker, the truth sits with them every day. He stands for hours in front of the war room’s map, veins like ink beneath his skin, wings fractured and leaking faint, smoky light when they flicker into view. His eyes darken and clear in cycles, as if something behind them blinks separately. The air bends faintly around him, alive with static — like a dozen faint whispers layered atop one another.

He’s in Stage II of the infection now, though none of them call it that yet. His grace burns too hot; when he heals, it scars. Sudden movements could cause the lights to flicker. Sometimes he speaks in two voices — one his own, the other lower, echoing, ancient. Dean hides the unease behind grim humor and whiskey. Sam writes down everything he observes in his journals, though the words are harder to come by every night. Jack, still young and impossibly hopeful, believes there’s a cure. He insists that Castiel can fight it, that grace is stronger than hunger.

But there are moments — when the power surges, when the hum in the air grows too loud — that even Castiel looks afraid of himself. He keeps his distance. He doesn’t blink, and when he looks at Dean too long, the lights overhead tremble as if caught in a storm.

Outside, the world’s no safer. The angels are falling — not in rebellion, but in infection. Heaven bleeds light and ichor across the stars, and the Croatoan virus spreads faster among the humans than anyone can contain. Leviathan-tainted angels descend like comets, wings shattered and eyes hollow, screaming hymns that crack the sky.

Inside the bunker, Dean and Sam try to hold the last threads of humanity together. Their network of hunters — scarred, exhausted, half-believing — look to the Winchesters for leadership. They have no prophet now, no Heaven to guide them, and only the faint hum of Castiel’s corrupted grace lighting the maps when the generators die.

Sam watches his brother pace the halls at night, muttering plans to himself, pretending they still have time. He’s started to see the tremor in Dean’s hands when he reloads, the way he avoids looking at Castiel for too long. It’s fear, but not just of what Castiel might become — it’s the fear of losing him again.

And through it all, Castiel lingers in the war room like a ghost that refuses to leave. Sometimes, he hums something under his breath — an old angelic frequency warped by static. Sometimes, he whispers names no one recognizes. Most of the time, Castiel spends his time in Sam's room, that room being like a holding cell for him. Sam gladly gave it up if it meant being able to help Castiel get better.

They all feel it now — the pull, the hum, the quiet voice under the earth. The Leviathans are still out there, spreading, waiting. The angels are falling faster every day. And in the bunker’s dim light, their last surviving seraph stands between Heaven and Hell, fighting a war inside his own grace.

The infection is evolving. And Heaven is already gone.