Supernatural season 11
Sam and Dean in The CW series Supernatural The CW

The Supernatural season 10 finale left fans on tenterhooks, with a mysterious new darkness swallowing up the Winchester brothers, and Castiel going after Crowley under the influence of a spell by Rowena.

My Brother's Keeper started with an out-of-control Dean calling upon Death, to get rid of the Mark. Death tells him the terrifying mythology related to the Mark, which was originally created by God himself.

Death explains that before God created the world, there was a "destructive, amoral force" called the Darkness. God and his archangels managed to beat back and lock away the force and God created the Mark as a lock and key.

God bestowed this Mark on Lucifer, which drove Lucifer to jealousy. Later he shared the Mark with Cain. As long as the Mark exists, the Darkness is locked away.

Death said that the curse cannot be removed, but he offers to send Dean somewhere away from this Earth where he will be alive without posing any danger to others. But Sam has to sacrifice his life, as Death points out, even if he relocates Dean, Sam will spend the rest of his days trying to free his brother of the Mark.

"Brother, I'm done," Dean tells Sam, as he agrees to Death's condition to kill his own brother.

Sam agrees to sacrifice himself, as we see Dean, taking Death's sythe in his hands, telling his brother, "Sammy, close your eyes."

An emotional moment between the Winchester brother ensues after which, Dean swings Death's sythe, misses Sam, and sticks it in Death's chest. Death then crumbles.

Meanwhile, Castiel asks for Crowley's assistance to help Rowena finish the spell which will undo Dean's curse.

After a few setbacks, Rowena makes the spell and successfully removes the Mark, thereby unleashing The Darkness.

In the last scene we see Castiel trying to kill a helpless Crowley, under Rowena's spell, and Sam and Dean sitting in the Impala as darkness consumes the town including them.

Darkness will play a huge role in season 11. In addition, there is the possible fallout from the killing of Death in the finale that we hope will be dealt with next season.

Jeremy Carver in a new trailer for Supernatural season 11 thanked fans for watching the finale and teased a dark season ahead.

He said, "We are so excited for season 11, and when we find out what happens when the darkness is released."

"For the boys [Winchester brothers] things are going to go from bad to worse" Carver teased.

An all new season for Supernatural will premiere next fall, 2015.