The season ended with Lucifer and Mary in another reality, Castiel apparently dead, Crowley definitely dead, and the nephilim child of Lucifer alive.
In Season 12, they ended up battling enemies on two major fronts: Lucifer with his demons on the one side, and the British Men of Letters on the other.
If fans know one thing about Supernatural after all these years, it has to be that the show does not hold back from going in wild, crazy, and very big directions.
While Sam, Dean, and Mary did need to work with the Brits in order to track some big baddies, they ultimately betrayed the Winchesters and brainwashed Mary to murder every American hunter she possibly could.