Many people were surprised when the Season Ten finale did a one-eighty on us, Death revealing that the Mark of Cain was all that stood in the way of the release of the Darkness- a pre-Biblical force of chaos- being released into the world. After the episode ended, there seemed to be at least one person on every chat thread protesting that the Darkness was a last-minute insertion, an excuse for a Season Eleven.
But there are instances of foreshadowing earlier in the season. The most striking instance- the only one that I’d swear to- comes in Book Of The Damned, where, describing how the Book was made, Charlie says ‘a nun locked herself away after having ‘visions of darkness”- the BoTD was what she emerged with.Could the Book hold information about the Darkness, then? If so, well- Rowena has it, so Sam and Dean are screwed. Must be Tuesday.
Then there’s the fact that the Season Ten premiere was called Black, which just seems a little coincidental given the season’s ending. People assumed that it referred to Dean’s demony eyes, but it could also have been foreshadowing. Ah, cyclical storytelling.
The last possible instance that I picked up on, though no doubt there are more, was in Girls, Girls, Girls, when Dean told Cole, concerning his own ex-demonhood, ‘once you touch that kind of darkness, you don’t ever come back from it’. As this, like example #2, is in connection with Dean’s demon phase, perhaps the Darkness has something to do with demons and their origins. And the mass of Darkness that swallowed Baby did look very like demon smoke- so it looks like we’ll be going deeper into what makes demons demons.
My point is that the ending, while unexpected, did not come out of nowhere. The showrunners would probably have had two different endings in mind for Season Ten- one for if the series ended there, and one for if they were picked up for a Season Eleven. And as they were picked up, they opted for the Darkness.
Personally, I welcomed the announcement of Season Eleven as good news- it’ll be my first in real time, and we’re having a femme fatale, old crap come back to haunt Sam and Dean, a Baby-centric episode, a possible Sam love interest, and maybe even Dean in a Hawaiian shirt.