So to close the gates of heaven/hell, you need one person on the outside, the Earth side, and one person on the inside, doing something at the same time. Somehow Sam ends up in Heaven and Cas ends up in Hell as the Gates close.
(it was supposed to be Sam inside Hell gate Dean outside it, Cas inside Heaven's gate & Metatron outside it—because Metatron doesn't want to be in Heaven—perhaps he tricked Cas somehow, told him there was a way out or something.) Crowley is on Earth when this happens and is stuck outside Hell, while Abaddon is inside it, and she becomes queen.
Castiel turns around from the closed gate to see Abaddon and the other demons surrounding him. He's been in Hell once before, but that was leading a garrison of angels, tearing a swathe to the righteous Man. Now he's alone, outnumbered, and weakened by torture and the angel-bullet wound in his stomach.
They all rush at him, clawing at him (since this is Hell, all physicality is an illusion—they're not tearing into his vessel but himself) and they drag him deeper and deeper and deeper. There are seven layers—the deepest is the Pit, but nobody goes in there because it's not connected to the rest. Think of it as a photoshopped painting with seven layers in it, or a printed page of pictures—you put the cyan on as one layer, the magenta as another, etc—but they are all the same picture, just different, deeper, parts.
So he's finally so far down that he's all alone, but the clawing doesn't stop. Hell itself is trying to negate him—he belongs to something utterly foreign, and all his willpower is not enough to stop it from reaching out and trying to tear him into shreds, into oblivion, into nothingness, his very being.
This goes on for a very, very long time. Time goes slower in each, deeper layer—but in the deepest layers—seven, six, five—there is no time, only eternity. Eventually, almost unable to keep going, starting to have his essence dissipating against his will, he remembers Gabriel. How he hid his Grace under a mask of Pagan magic so no one could find him.
He could do that.
So he pulls a little of hell to himself, fashions it around himself like a shield, and curls himself up inside it, draws into himself inside it. But it is attacking him, worming itself through the holes, and he can't keep a hold of it because it won't stay there. It's repelled, it wants to annihilate him, it won't act as a shield against itself. So he pulls it further to him, touching his grace, forces himself into it and it into him. It's like holding together two same sides of a magnet while being attacked by killer eels, but suddenly something lets go. The shield grafts onto the outside of him, flowing into him, a camouflage.
For a moment.
But it knows he's still there. His Grace burns too bright, even hidden—it can feel him. He pulls more and more around him, a barrier that can't be crossed, but it's sneaking through when he turns his back, it's a trickle of water, a tide against a castle of sand.
He needs to make a better shield. How? He already knows making it thicker isn't working—he needs to dim his light more. He needs to pull it further in.
It works. He is fighting for his life, pulling Hell into himself and putting himself back into it, twining himself around his shield tightly, smoothing the edges, and then, desperately, letting it bleed together until you can't tell the difference between them.
And then it stops.
Slowly, conscious thought returns.
He is alone, and Hell—even when it's not trying to destroy him—is not pleasant. It is the very definition of unpleasant.
Eventually, he moves. He starts to hear—to feel—something—souls, screaming. If he can get out of the barrier, of formlessness, further up, he can get out. He struggles for infinity, but manages to get out—all of a sudden, there is boundaries. Definition. And screams. They are not sound—it is more like he feels them. He wanders toward them, because for so long, the universe has been unchanging, and this is something different.
And then he sees a demon.
((What are you looking at?)) It isn't speech, but it's communication—he feels like he's trying to speak a foreign tongue he once knew but has almost forgotten, manages to get out, somehow, ((nothing.))
It is torturing a soul, a screaming soul, it screams with pain and anger and fear and despair—tearing into it, tearing it apart.
Castiel watches. There is, after all, not much else to do.
((You're not doing a very good job)) he says eventually.
This angers it. Its horrors rise up in defiance. ((Oh yeah? Think you can do better?))
He looks into the soul. It was going about things in the wrong way, that was all. ((Yes.))
Hell was all about pain, but pain could be withstood. On the other hand…
Castiel reached inside the soul. It flinched at his touch, but its light seemed to suck at him hungrily, as though sensing a kindred spirit. ((It's all right.)) There it was—all the hopes, the fears, the despairs, the self-hatred and the hatred of everything else—all the things that Hell was made of, sustained, kept at bay, only by a tiny thread of defiance. Of hope.
He reaches to it, nudges gently, pushes things together touches with a light, delicate touch—and the soul collapses in agony and despair, its light dimming, almost put out.
((You should probably be able to handle it from here.))
He leaves. He wants to get out—that his only mission, that is what sustains him, when he can't remember anything else—he knows that something of great importance—nay, something of more importance than anything else—lies outside.
The journey to the next level is excruciating, but he perseveres. Exhaustion threatens to overcome him, but he's been this far, he can go a little longer. He breaks into the next level.
The screams are louder now, reverberating, echoes, tears and spilled blood. The demons congregate here, spend their time, pass away the nonexistent millennia. He does not dawdle—he has only one mission, and that is Up.
The way to the next level here is easier—the barrier is in distraction, but he is not easily distracted by pleasures or pain of any sort. He passes through.
The level after that is strange, quiet. He has gotten unused to quiet, the constant monotony of souls—but this is Despair, and Loss, and everything is silent and still and empty. He cannot find the way. Before, the path had been easy to see, but this is a twisted labyrinth made of mist and darkness. It goes on endlessly, looping in on itself. He needs to ask for help.
[How do you get out? Do you know?]
[I might.]
[What do I do to get out, which way is it?]
[Easy—if you pay the price.]
The demon is disgusting, despicable, it feels of all the slime and mold and the smell of dead things.
[Make me feel alive.]
It is like being attacked in the deepest layer from last—but this time, he is not fighting back, cannot fight back. The seep of water through a brick wall a wooden wall a paper wall—being pulled, falling down, forced into shapes he cannot make—it flows into him and all his instincts call for him to throw it out. It is not that the feel of Hell clings to it like a persistent scent—everything smells of Hell in Hell—it is not the fact that it is horrible and wrong—everything in Hell is horrible and wrong.
It is losing control.
It feels alive, he feels drained, sucked dry, dead. He cannot tell who is who anymore, where the boundaries start and end. Disgust and ecstasy mix until he can't tell which is which anymore.
He is nothing.
And then it is gone. Castiel pulls himself in, forms a shape for himself, feeling fragile, dirty, empty. The other is glowing with life, energy, but already, it begins to dim, sucked out by its very nature.
He knows the way out.
In the next level, the sense of time that has been so slowly building, abruptly returns, and with it, order, causality. His wings are corporeal, and so is everything else. This level is physical. This level is a dungeon, filled with human forms. He stares, fascinated, as he passes, the sensations against his skin, the colors in his eyes, the shapes and forms. It is beautiful.
He stops at one of the cells. "Hello."
The frightened girl only inches back, not meeting his eyes—she does not answer.
"Who are you?"
Still, there is no reply.
He reaches in through the bars, grabbing her by the neck, feeling strong hands bruise thin flesh—"answer me!"
But she only screams and sobs.
He drops her to the floor.
And keeps walking.
He picks a stone off the floor, heats it red-hot, rolls it between hands that don't blister. He throws it at another one, but he doesn't so anything.
This level. So medieval.
And no one will talk to him—they are either too stubborn, too frightened, or too mad. The ones that do talk are incoherent, and there words are not meant for his ears.
The way up from here is through a door. The heavy iron trellis of a dungeon slams shut behind him, and he smiles.
He is in a long, dull corridor, and he is in the shape of Jimmy Novak.
It is all there—every last detail, even the trench coat, though, he realizes in mild annoyance, that is black for some reason, and nothing he does will change it back.
The numbers at the end of the line go up by one as someone appears. They are here, in an unchanging line, an unchanging corridor.
This is Hell. This is hell, as much as the medieval dungeon, the place of despair, the fabric trying to rip intruders apart. It is all the same, the levels—are only deeper in. truer. The same souls which, here, stand, unutterably bored, are being tortured with archaic instruments, by demons, by Hell itself.
He has been here once before, but then, he couldn't see all the way down.
He walks.
"How do I get out?" Castiel asks, but no one knows the answer. He is so close—this is the last level, but the gates are sealed tightly, and he can't find his way home.
He finds a reaper. "Do you know the way out?"
"There is no way out," The reaper returns, bored. He probably gets inquiries like this a lot, Castiel reflects. No one, not even demons, like being in hell. "Now, I've got to be going now, so if you'd excuse me—"
Castiel's angel blade is in his hand. The reaper stares at it in ill-disguised fear.
"Show me the way out, and you can have this."
It takes him to a hole.
"I told you, it won't do any good anyway, only humans can pass through the portal."
Through the hole, Castiel can see the grey forest of Purgatory.
"This is not Earth."
"I never said it was Earth. There's no way to get to Earth from Hell. The gates are closed."
He reaches out with the hand that holds his blade. Angel blades can kill everything except for death—perhaps it can also rip a hole between worlds.
The reaper watches as he cuts a thin line in the air.
"Fine, I got you out, now I want the blade."
"Not until I'm on Earth again."
The reaper doesn't argue. He's the one holding the knife.
He steps into Purgatory, concentrates, and with a rush of displaced air, appears at the other portal—the way to Earth.
He reaches out, slowly. Cuts a hole in the nothingness. Steps out.
The reaper is waiting for him on the other side. "I got you out."
"That's right," Castiel answers.
He's standing on Earth. The sun is shining, there are smells of flowers and trees and grass and dirt and tar and gasoline.
He's on Earth.
He turns to face the reaper, smiling. He feels almost giddy, he feels… happy.
He steps up him, stabs the agent of death quickly in the gut. It dies fast.
"But I can't let you take my sword," he finishes, quietly, and wipes the blade on the ground.
To his annoyance, his trenchcoat is still black. But that is a minor annoyance.
He's on earth. All he has to do now is find Dean.
.
.
.
I don't remember where I got the idea for Cas's trenchcoat being black, but I remember reading the idea, that what if he wore a black trenchcoat in season 9? which made me start to wonder how that would happen, so that idea was what inspired this story.