Meanwhile, Sam (Jared Padalecki) and Castiel (Misha Collins) make Rowena (Ruth Connell) cast a spell to destroy the Mark of Cain.
Castiel enters her mind to see her greatest love is Oskar: a boy she befriended 300 years ago and granted immortality.
Dean arrives at a bar where he summons Death (Julian Richings), asking the Pale Horseman to kill him as the Mark is affecting him severely, using a smorgasbord of junk food as enticement.
In revenge for making her kill Oskar, she casts a spell on Castiel, causing him to attack an immobile Crowley.
As Sam and Dean leave the bar, lightning begins striking multiple places in the ground, causing columns of black smoke to roll into an enormous force, The Darkness.
Amy Ratcliffe of IGN gave the episode a "great" 8.8 out of 10 and wrote in his verdict, "The Mark of Cain business is finally resolved.
While it's disappointing that no one else mentioned the Darkness (like Cain), it sets up a big bad for Season 11 and the Winchesters need to unite against a common enemy right now and rebuild – after they get out from under that inky cloud.
Or, it could get bogged down in the execution of so many massive and far-reaching plot threads, and lose grip of many of the storylines, like what happened this season.
And yet, that was only one of the memorable moments in tonight's beast of a finale, which once again did the thing that this show does best: Put a Supernatural twist on a Biblical story.
"[9] Sean McKenna from TV Fanatic, gave a 4.6 star rating out of 5, stating: "So many questions and possibilities for Supernatural Season 11 have me pondering about what just happened and what's to come.
"[6] Bridget LaMonica from Den of Geek, gave a perfect 5 star rating out of 5, stating: "The episode asks the hard question: 'What is evil?'
Has all the good the boys have done over the years - y'know, saving people, hunting things - make up for all the mistakes they've made as well?
The people who died by accident, the guy whose soul Sam sacrifices, and more recently the heroic death of Charlie.
I readily admit that I'm a cynical bastard who routinely falls into the trap of Supernatural's finale hype-machine only to be disappointed when it fails to meet expectations.
After a bumpy season full of awkward pacing, dropped storylines, uneven characterizations, and the occasional bout of predictability, my hopes for 'Brother's Keeper' weren't at an all-time high.