The Winchester brothers might lose their mother again, in the finale episode of "Supernatural" season 12. Meanwhile, a mysterious character appears that brings Castiel (Misha Collins) in for a surprise.

In the upcoming two-part special season finale of "Supernatural" season 12, the fight for Lucifer's (Mark Pellegrino) unborn baby and the feud between the American hunters and the British Men of Letters will finally come to an end.

In the first part titled "Who We Are," Sam (Jared Padalecki) and Dean (Jensen Ackles) will have to rely on themselves in fighting the British Men of Letters, since their own mother — Mary (Samantha Smith) — is now one with Ketch (David Haydn-Jones), done from all of Toni's (Elizabeth Blackmore) brainwashing.

However, it appears that the Winchester brothers have taken someone from the British Men of Letters on their side. As seen from the season finale promo, it appears that Sam, Dean, and Toni work together to escape the bunker before they all die inside.

In the following hour with the episode titled "All Along the Watchtower," Lucifer, the Winchesters, and Castiel all fight each other to win over custody of the devil's unborn child.

Since Lucifer's freedom from Crowley (Mark A. Sheppard), it is possible that he already has a plan in winning custody over his child, as he tells Lucifer at the end of the finale promo that he has lost.

Meanwhile, the post-production team of "Supernatural" launched a teaser for the upcoming season 12 finale, which might be hinting at Mary's death.

In the teaser provided by Entertainment Weekly, Dean is seen crying, while Mary is almost nowhere to be found in the teaser. It is possible that Dean is crying for losing Mary for the second time in his life.

Also, the teaser hints at a mysterious character that appears to be someone Castiel knows. When this mystery character walks up to Cas with a gun in his hand, the fallen angel says, "You?"

The two-part special season 12 finale of "Supernatural" premieres on Thursday, May 18, at 8 p.m. EDT on The CW.