The Doctor hustles him into the TARDIS, panicking, pallid and shaken to the bones, because Cas did it—he opened the doors to Purgatory, absorbed every monster they ever fought off together, pretended he was a god while the Doctor watched and chewed his nails to the quick. Cas made him promise not to interfere, without knowing the number one rule: the Doctor lies.
He lied so badly, as soon as the Winchesters fell to their knees, he gave them a harried apology and wrapped your arms around Castiel, reminded him who he was and what the two of you had—our girl, think of our girl, Aliana, think of how much she'd miss you—and the Doctor ran like the soles of his shoes were aflame, like every bad thing hiding in the corners of the universe was chasing them, like he was a young man fleeing his oppressive home planet.
The Doctor seals the TARDIS doors in hopes of keeping the demons at bay, and jumps into the Time Vortex, telling the TARDIS she can go wherever she wants—Venus, Raxacoricofalipatorious, Gallifrey, anywhere but Earth. She obeys like she always (usually) does, and the Doctor thinks himself and his family safe. We're safe. Castiel is standing dazedly by the door, Aliana is running towards the console room right now, the little pitter-patter of her feet helping you breathe, and we're safe.
They're not safe.
The Doctor feels it before he sees it—slimy and chilled against his skin, eliciting causeless goosebumps, tickling the back of his neck and breathing heavily near his ear. He knows that sensation, that scent. Leviathan. He looks to Castiel in alarm—How did one of them get in?—and he sees Cas's frozen, expression carved by terror, staring at Aliana, who's just arrived and is crying, she's so happy to see her fathers home again.
Her face turns and suddenly she's up to her ankles in the goo the Doctor felt, the very goo that's bubbling out of Castiel's pant leg and making him unfurl his wings in instinctive self-defense. The Doctor breaks the tableau and runs towards her, almost slipping and catching himself on the console, but he's too late—
Her clothes are stained and her hair is drenched in sweat and she's bursting at the seams with black, eyes still Castiel's blue but skin marred with obsidian veins. Cracks are forming in her skin and blood's leaking through, trying to escape. She's going to burst, and you know it—your daughter is going to burst and all at Doctor can do is clutch at her shoulders and shake her, yell at her.
"No! No! This wasn't supposed to happen!" They're both crying now, the Doctor and her, and she's staring up at him, as terrified as he is, soundlessly asking for help.
He feels it before you see it—Castiel's wings brushing his back, pulling the Doctor away as Aliana goes blank, and then erupts into a jagged grin. She's not her anymore. She's gone.
"This vessel's so small, I feel like I've eaten a planet. Of course, you know how that feels, don't you, Castiel?" She's gone, but her voice still resounds around the room, tinny and stretched thin.
Castiel steps forward and he's quivering with power, wings twitching and sharpening before the Doctor's horrified eyes. He runs towards him—running, running, always running, too cowardly to save his family—and Castiel slams him back into the wall with strength borrowed from monsters, turns his head for a moment to meet the Doctor's eyes, and he knows what he's going to do.
"No, no, no, Cas, no—"
"Do you want her to die?" Castiel barks it at him, and the Doctor almost doesn't recognize him under the shroud of anger, eyes crazed and mouth set in determination.
"Are you going to exorcise me again? Oh, but you set me free!" The Leviathan laughs with Aliana's voice, and the Doctor silently pledges to get Castiel back, no matter the cost. "You know, your daughter's still in here. Kicking and screaming. Calling for her fathers. Which is rather funny, since one of them lead me to her—"
"Take me," Castiel interrupts, and suddenly the Doctor's crying again.
Castiel's too good of a father, he realizes. He's too loving, too selfless. Of course he's going to get himself killed. Of course he's going to insist on being the martyr. Of course.
It shrugs. "If you insist, Daddy-O." The goo pulses out of Aliana, leaves her limp and white, her hair fanning out as she hits the ground, auburn accentuating crimson stains. Castiel shakes and the Doctor rushes over to their little girl, because Castiel just sacrificed himself for her, and the Doctor'll be damned if he's going to let her see what he'll become.
She stirs when he picks her up, brow furrowing and eyelids twitching, so he hides her from Castiel's convulsing form and presses her head into his shoulder, tears staining the ripped tweed.
"I'm sorry, baby girl," he whispers, kissing her head and running from the monster through the endless TARDIS corridors, wishing a hole would open up and they'd both be engulfed in darkness, because he never, ever wants to see Cas with black veins, smirking, murdering.
"I'm sorry I failed."