Supernatural season 11
Dean with Castiel in Supernatural season 11 episode 11 promotional pictures The CW

Supernatural season 11's all new episode will find the Winchester brothers — Sam and Dean — worried about Castiel, who is actually Lucifer. Episode 11, titled Into The Mystic, will air this Wednesday 27 January at 9pm EST on The CW.

According to the official synopsis: "Sam (Jared Padalecki) and Dean (Jensen Ackles) investigate a case where people are suffering violent deaths after hearing a mysterious song." Click here to watch the episode online on the official CW website (only in the US).

In the midseason premiere, Castiel made a deal with the devil and became his vessel after he promised help to deal with Darkness. Dean and Sam are unaware of it. However, Dean did find something odd about Castiel. Will he find out the angel's devilish truth in the episode?

The promo opens with Dean telling Sam: "We need to find us a case." When Sam asks: "What about Cas?" Dean replies: "There is something off about him."

The promo shows Castiel walking down the forest as an angel shows up and says: "Lucifer?" Castiel kills him with the stroke of his finger. The voice-over says: "The Devil you know gets stranger."

Showrunner Jeremy Carver said with Lucifer and Amara at large, the Winchester brothers will fight all threats unitedly. Carver told Entertainment Weekly: "I think what we were saying at the beginning of the season is that the boys really do have the same want this year. There are always secrets in the moment, which we've seen. I think ultimately, we've seen each brother come around to admitting the truth. I don't see necessarily a fracture in the relationship coming."

"I think there's real differences of opinion based on real vulnerabilities that each brother is willing to share with the other. It feels like it's not quite as dastardly or dark as Sam stealing off with Ruby in the middle of the night. It's a bit more of, at least for the first part of the second half of the season, a bit more of a united front than we've seen, which we like," he added.