I’m sitting here waiting for a video of mine to render, so I will answer this ask too! I usually don’t read too many dark ones, but I do love me some scary, disturbing fanfiction every once in a while.
And hey, you asked for dark, so there’s lots of non-consent issues and things like that. Also very disturbing imagery. So.
Folie à Deux dean/castiel, pg-13, 5k
Summary: A madness shared by two. (The AU where Henrickson was right: there is no thing as the supernatural, John was a paramilitary whackjob, and Castiel is Jimmy Novak’s other personality.)
And I Will Walk On Water dean/castiel, nc-17, 122k
Summary: In which Dean discovers that the archangels didn’t finish their job after all, and that demons picked up where they left off.
The Reality of the Thing Seen gen, r, 3k
Summary: Dean’s not hungry, but he’s got flesh covering his bones and he’s got blood running in his veins.
bang bang (my baby shot me down) dean/castiel, pg, 4k
Summary: Something shatters and the world goes red.
Let Him Kiss Me With The Kisses Of His Mouth dean/castiel, nc-17, 6k
Summary: After running away from demons’ trap, Dean and Cas hide in a church. Sadly, Asmodai, demon of lust, knows exactly where they are and casts a spell on the church. And things get a bit… hot inside. For Dean, anyway. Dean/Castiel. First time. under-a-spell!Dean fucking virgin!Cas. Rough sex, may be a bit bloody, Dean fucking Castiel from behind with Cas leaning on the altar. Bonus points if Cas pleads Dean to stop at first and then comes to really, REALLY enjoy it.
Dormant Siren dean/castiel, nc-17
Summary: The bar erupted into applause again as the dying chord rang out and Castiel dared to open his eyes and turned to see Dean. Castiel bit back another desperate, wanting sound. “One word, Cas, and he’s ours. I could have him disappear without a trace by midnight,” Gabriel growled, and Castiel bit his lip again, and nodded.
To Reign in Hell dean/castiel, nc-17, 70k
Summary: When Castiel abandoned Heaven to be with Dean, he damned himself to hell. Now, centuries later, Dean is sent on orders from Heaven to stop a demonic Castiel from leading the armies of hell in another war.
The Forty-First Year dean/castiel, nc-17, 4k
Summary: Dean was not nice in Hell. There’s non-con and torture and all sorts of nasty things.
The Beating of His Hideous Heart dean/castiel, nc-17
Summary: Dean is the charming sociopath and Castiel is the fresh-faced prison chaplain, dub-con. With Dean, after weeks of working Cas over, finally getting his hands on him after a careless guard leaves them alone.
The Melting Verse dean/castiel, nc-17, 110k
Summary: The epic love story of Dean and Castiel told in 110,000 words. Stuffed to the brim with angel!porn, whump, angst, action, wing!porn and occasional soppiness, and Sam doesn’t get neglected either. (Although he doesn’t partake in the angel!porn, I should probably add.) Some of it’s really, really dark (with non-con), so be prepared for that if you’re just looking for a little schmoop.