"Supernatural" season 10 finale ended with a big bang, as usual. The Winchester brothers tried to keep each other from dying, but ended up unleashing a whole lot of evil into the world they are so desperately trying to save, as usual. But instead of angels and demons that try to spill the Winchester blood, it is a mark that ruins the lives of Sam and Dean this time around.
In the "Supernatural" season 10 finale episode "Brother's Keeper", Death (Julian Richings) gave Dean (Jensen Ackles) a chance to save himself and the rest of the world from the Mark of Caine. Death explains that the Mark works as a lock to the prison of a pre-God evil called "Darkness".
If the Mark of Caine is removed from one's body without being passed on to another, it will unlock "Darkness". It remains unknown what it means, but the name suggests nothing good.
However, the only way to ensure Dean remains locked away from the world and Sam (Jared Padalecki) doesn't try to bring him back, is for Dean to kill his younger brother. Both brothers seemingly accept the fate, but in true Winchester style, decide to make the foolish choice of keeping each other alive, and killing Death itself.
Simultaneously, Rowena (Ruth Connell) performs a spell to rid Dean of the Mark and thereby unlocks the prison of "Darkness". This makes Rowena powerful enough to curse both Castiel (Misha Collins) and Crowley (Mark Sheppard), thereby getting the Angel to kill her Demon son. The scene ends with Cas almost killing Crowley, but if you know anything about "Supernatural", you know that no beloved character is really dead.
Crowley will either find a way to rid himself or Castiel of his mother's curse or be resurrected in a Season 11 episode. It remains to be seen which of the two possibilities will happen.