Loyalty will be your undoing.

'Supernatural' Season 10 Spoilers: 'Deals With The Devil' Start To Form In Sneak Peek, Sam Recruits Charlie For 'Dark Dynasty'

It's a lesson Supernatural's Winchester brothers can never quite learn, and one Charlie will start to understand in "Dark Dynasty."

Sam (Jared Padalecki) and Dean's (Jensen Ackles) familial loyalty has taken them down some dark roads, and it's Sammy's devotion to his brother that will force Charlie (Felicia Day) and Rowena (Ruth Connell) to team up on the Book of the Damned. Charlie, too, will follow the Winchesters to the ends of the earth, despite Rowena's warning about just where such fidelity can lead (see video below).

'Supernatural' Season 10 Spoilers: 'My Brother's Keeper' Plot Revealed, What's In Store For The Finale

"This episode is where we really start to see Dean getting even worse," executive producer Jeremy Carver said of "Dark Dynasty." "Sam, even more desperate to help Rowena find a cure for Dean, is going to enlist the help of Charlie to help her use what is now the Book of the Damned and the codex and find some way for them to work together to find a way to actually decipher this spell."

Luckily, Cas (Misha Collins) will be on hand to make sure the techie-hunter and Crowley's mum won't go 12 rounds over the codex. Unfortunately, Rowena's warning seems like a dire bit of foreshadowing for Day's fledgling hunter.

Find out more when Supernatural airs Wednesdays at 9 p.m. ET on The CW.