Interdimensional fuckery

Mary Winchester has just finished a hunt when she sees him. He's a boy of fifteen, with curly blonde hair and white skin, patterned with freckles.
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The fact that Walt Stone has woken up in an alleyway that he's never seen before is alarming, especially considering he does not remember falling asleep in said alleyway.
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Jack stands up and looks around and then he's Very Aware of what's wrong as a tear in reality opens up, spits out a girl, and closes again.
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Alex is prepared to climb a tree. She is not prepared to fall out of one.
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Leo's day started suspiciously normal. And not the sort of normal where you go through your routine with a couple bumps in the road, but the kind where you did everything right. Of course, this meant Leo was waiting for everything to go Very Wrong.
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For some reason, three demigods, an antichrist, and a god's host all fall into a new dimension. Nobody knows what they're doing, least of all the author.