This week, "Supernatural" fans will see the Winchester brothers take a break from all things Mark of Cain, and instead tackle a case involving the mysterious disappearance of Amelia Novak, who has yet to resurface years after she went missing. 

Consumed by guilt of what he's done to the Novaks, Castiel goes to Dean and Sam for help after Claire, the daughter of the missing Novak matriarch, gets a new lead on her mother. By reuniting the mother and daughter, Castiel believes he would be making reparation for what he did to them in the past. 

But before the trio gets on with the task, they have to deal with a rampaging heavenly assassin that's killing everyone in its way. In the promo for the upcoming episode titled "Angel Heart," the hunters' pursuit leads them to a more horrifying creature — the ultimate monster. 

But the ultimate monster is not what they expect to be. The show, without a doubt, loves its own shockers and surprises and the identity of this feared monster is one, and it's a really big one. 

Dean and Sam aren't ready to learn that the ultimate monster is a teenage girl, and Sam quickly realizes she's got a mouth on her after she shamelessly calls him "pretty old," leaving Dean with a crisp "How dare you?!" response. She is not the ultimate monster for nothing, apparently. 

So in a nutshell, the ultimate monster is the same person that needs their help, Claire. As Dean and Sam get on to save Amelia, they do not realize what horrors her captors can do, enough to make them think Claire's happy ending is far from reach. 

It is still unknown what happens to Amelia. Although alive, her appearance and her current location have lots of stories to tell.

"Supernatural" season 10 episode 20 titled "Angel Heart" airs on April 29.