Kristin Dos Santos talked a little about the series in her new spoiler column in answer to a fan question.
Greg: Is there going to be a big bad on Supernatural this season? What is up with Castiel?There are going to be multiple big bads. Big old big bads. Aka leviathans. And Castiel is the one who accidentally releases them into the world. Translation: This is not good for Sam and Dean.
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