Sam and Dean are all out of cases to investigate in the next episode of "Supernatural" season 11 titled "Into the Mystic." In a bid to find one, they end up getting entangled in a new mystery involving Castiel's sudden change in behavior.

The promo for the upcoming episode shows Sam suggesting to his brother that "something is off" about their angel friend. Needless to say, he was right. This is because Castiel has become Lucifer's new vessel.

The "Supernatural" season 11 clip also hints that Dean will also eventually notice Castiel's behavior and he will be the one to "keep an eye on him" upon Sam's request.

While it looks like the Winchester brothers will be aware of Lucifer's masquerading and Castiel's secret in due course, there's the question of when that will happen and how they will be able to save Castiel and the other people Lucifer will potentially murder.

This won't prove easy. While they were initially out of FBI-related things to do, Sam and Dean will, in the end, have a lot on their hands in this episode as they will have to investigate brutal deaths instigated by a mysterious song.

Meanwhile, executive producer Jeremy Carver recently spoke to Comic Book Resources about the future of the characters and the direction "Supernatural" season 11 is headed.

Carver teased that Sam will not be happy upon knowing Dean's love affair with Amara. As for Dean, he will see this romance with God's sister as a weakness. However, it gets weird for him because he actually likes being around Amara.

"He has to make a decision whether to keep it a secret or not. It's a big moment for him to share his greatest weakness with Sam," Carver said of the ordeal.

Meanwhile, the other half of "Supernatural" season 11 will also see "winds change" for Crowley, who has been teaming up with Sam and Dean after being their number one nemesis.

"Supernatural" season 11 episode 11 "Into the Mystic" will Jan. 27 at 9 p.m. ET on CW.