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As soon as Dean tells him the book is calling to him, Sam mentally goes over the spellwork of Magnus's he has on his computer.
He's been dabbling for a while, easy enough to cast little protection charms and locator spells here and there. There's bigger stuff in Magnus's files, though, and he has them all on his computer just in case he needs to put something together in a hurry. He hates that he's right about that, but really, with their lives, it was just a matter of time.
There are several spells that might work, but the one Sam chooses is the spell Magnus used to hide his house. He knows now that most of the magic is having the right words and the will to do it; he's substituted just about every ingredient in all of the spells for other things and as long as his determination is on track – and it's always on track – the spell works. It works better with the right ingredients, but in a case like this, he only needs it to last until they get back to the bunker, when he can re-cast the spell with the right ingredients to strengthen it.
The damn Stynes interrupt his research, and when Dean insists he burn the book, he has to cast the spell on the fly, hoping his memory is still the steel trap it used to be. He casts at the same time he trades out the book, his spell concentrated on the iron box. Dean is distracted by the fight, but if he can still feel the book through Sam's spell, it'll become obvious on the trip home.
Dean doesn't notice – at least not that he lets on – and Sam sits through what should be a fun evening with Charlie and Cas half-distracted by the fucking book. It might not be calling to Dean anymore, but Sam can't get it off his mind. He knows he doesn't want to cast anything out of it himself, but maybe, if he offers it to someone short-sighted enough to use it without thinking, they can get the curse off Dean and that someone – Rowena, his mind provides easily, and he trusts his gut on this one – will pay the price.
He knows they're never that lucky, but he's desperate to get rid of the mark before Dean goes dark side, and he has a few cards up his sleeve yet. Maybe he can turn this one to his favor. He casts a luck charm over the table as they eat their pizza. Even if he doesn't get out of this unscathed, maybe the rest of his family will.