In "Supernatural" season 11, Dean and Sam will be reunited to fight their biggest adversary yet – the Darkness itself, which will come in the form of a seductive and sinister alien named Amara. In order to defeat her, the brothers will have to get help from people from their past. 

Back during the San Diego Comic-Con, executive producer Jeremy Carver said that some of the entities the Winchesters faced and overcame in the previous seasons will make an appearance in season 11. This is not to bring them down again but to actually get their help in eliminating the new threat. 

"As responsible as the boys feel for having released The Darkness and how much they personally want to defeat it, they're not going to be able to do it alone. They're going to have to confront some figures from their past," Carver told TV Line. "They're going to have to make some unexpected and unholy alliances involving folks from their past, which will have personal ramifications, as well. Because these are some pretty significant figures from their past."

However, Carver did not get into detail as to who will actually reappear next season.  

The Darkness was set free when the Mark of Cain was released from Dean. Without a host to cling to, it was let out into the open with the potential to turn the world against itself. This new foe predates the brothers, the bible, Castiel and even Crowley, which is why it will be extra difficult to defeat it and why more individuals are needed to at least try. And Dean and Sam will stop fighting each other and start fighting alongside each other. 

"Supernatural" season 11 will premiere on Oct. 7 with an episode called "The Bad Seed."