'Supernatural' season 12 episode 15 spoilers, speculations: The hellhound returns

The plot of episode 15 of "Supernatural" season 12 temporarily veers away from the Winchesters' dealings with the British Men of Letters and their mother issues. Instead, it brings back a monster that the brothers — and the viewers — have not seen for a long time.

Rumors of a hellhound — or as Crowley (Mark A. Sheppard) called it, "the" hellhound — will be terrorizing a campsite on the upcoming episode of "Supernatural," prompting Sam (Jared Padalecki) and Dean (Jensen Ackles) to investigate the case. The demon's ominous description of the creature indicates that this is not a typical hellhound, and an ordinary one is already formidable and tough to kill as it was.

The trailer has led fans to speculate what differentiates this monster from the rest. Some believe that it could be the hound that killed Dean back in season 3, while others are saying it could be the first one that was made or born. While not tackled in the pilot, it can also be expected that Sam will talk with Dean about a possible alliance with the British Men of Letters, whom the younger Winchester pledged allegiance to in the previous episode.

The synopsis of episode 15 also revealed that two of Crowley's underlings will discover a secret that he is hiding, which could be that of him keeping Lucifer (Mark Pellegrino) a prisoner. Promotional photos showed the latter no longer bound by chains, which can spell disaster for Sam and Dean.

The upcoming episode will also show Castiel (Misha Collins) chasing after a lead about Kelly Kline (Courtney Ford). In episode 8, she became pregnant, possibly with a powerful Nephilim, with Lucifer while he was impersonating Jefferson Rooney (David Chisum), the President of the United States.

On another note, Castiel's killing of the Reaper Billie (Lisa Berry), done by the former angel to protect the Winchesters on episode 9, will have "cosmic consequences," according to Misha Collins in an interview with TVLine.

The episode, appropriately titled "Somewhere Between Heaven and Hell," will air on March 9, Thursday, at 8 p.m. ET on The CW.