They think he's dead, but he's not. He's just been smothered. The Leviathan took him in and swallowed him, snatched the reins from him, stole his voice and his family. And he can't do anything about it.
The Leviathan only lets him watch through blurry, cracked glass, and Castiel takes in every moment of it he can, because he misses his Doctor and their little girl so dearly that perhaps he can find the strength to break through. He's a miner, stuck in a hole, chipping away at the burnt Earth scorched by the molten lava beneath him, and he just wants to get home. He yearns to escape the Leviathan's clutches and be with his family again, if only for a little while, if only until the tides turn and Castiel's being smothered again.
It works, sometimes. He claws his way through, the Leviathan fighting him every step of the way, until suddenly the Leviathan lets go. His oppressive presence vanishes for a moment, takes residence in his gut, and Castiel can finally see clearly.
Every time, all he sees is his daughter. Aliana's bright, lovely face, smiling at him, holding his hand and talking about some dream or another, and Castiel's happy for the first time in weeks. That's his little girl, right there. That's Aliana. Safe.
The breaks in battles last longer than others. One time, he was allowed to be with her for an entire hour. He sat beside her and helped her draw the TARDIS, put his hand over hers and got the cracks in the wood just right. Kissed her forehead in grateful celebration.
He doesn't want to ponder why the Leviathan lets it happen. He's just happy it happens at all.
And then he starts seeing the Doctor.
He misses his Doctor, of course. He misses his Doctor's smile, and his unending enthusiasm, and his loving nature. He misses how the Doctor changes the experience of living, just by being near. When the Leviathan lets him through, lets him see the Doctor for the first time, he doesn't see a brilliant madman with a box and a family and a verve for adventure.
Castiel's sitting upright in their bed the first time, with a book in his lap and a steaming mug on the nearest nightstand. He automatically searches for Aliana without paying any mind to where he is—that's never been important before. Half the time he doesn't notice what he's helping Aliana draw. This time, though, his eyes rove around the room quickly, like a man taking a breath of air before submerging himself in the sea, and he's greeted by the now-unfamiliar sight of his Doctor on his side of the bed, slouched, presumably asleep.
Castiel's reminded of the many trite romance novels the Doctor encouraged him to read when his heart flutters—stops in shock and restarts with wonder.
He looks so old. So worn-down. The Doctor's sleeping with his chin on his chest, hands folded and resting on his sternum, lackluster hair flopping down and concealing his eye. Castiel looks closer and sees the Doctor's mouth twisted into a worried frown, finally spots the tension in his shoulders. He's sleeping like an animal of prey residing in a tiger's den.
Castiel reaches out to touch him instinctively, aching to soothe the pain away as he's done time and time before, but the moment his fingertips make contact, the Doctor jumps. He shrivels away from Castiel and his frown deepens. Whatever peaceful façade he'd held before dissipates.
"Doctor?" He's never spoken before. He assumed that would be more control than the Leviathan was willing to lend. He's willing to bend the rules this time. He wants to fix whatever damage has been done.
"You're still imitating him, I see." The Doctor's voice is low and scratchy, grating on Castiel's ears like wayward grains of sand. It doesn't sound like his Doctor at all. It sounds broken and defeated—everything Castiel could count on the Doctor never to be.
"Are you in pain?" Of course he is. You can see that he is, in his feral but-subdued-eyes (a tamed animal, a thing of beauty whipped into submission), in the tremor of not only his voice but his hands. His hands are shaking like they're trying to reach for something, whether it be a knife to vanquish or a person to hold.
Castiel's supposed to be that person to hold.
"You can stop now. You don't need to—to use him to control me."
Suddenly, Castiel's pulled back inside himself, back into the sea, back into the murky depths the Leviathan's made his cage, but he can still hear his Doctor, clearer than crystal.
He can hear the Leviathan, too.
"My dear Doctor." The Leviathan's voice is muffled, his face pressed into the Doctor's side. A feeling of warmth and disgust runs through Castiel, but he doesn't have the strength to climb anymore. "You could always tell when I was acting."
"I'll know my Castiel when I see him."