Supernatural (Season 7)

After absorbing all the souls from Purgatory and declaring himself God, Castiel goes forth to punish all the wrongdoers in the world. Very quickly, though, he discovers that he absorbed the Leviathans, who are attempting to break free of his vessel. Sam and Dean manage to return most of the souls to Purgatory but fail to return the Leviathans, who seemingly kill Castiel before vanishing. The Leviathans inhabit the bodies of many different people around the world. Sam and Dean learn that their weakness is Sodium Borate (aka Borax), though it has minimum effectiveness on the Leviathan leader Dick Roman. After Roman kills Bobby, Dean becomes obsessed with taking the Leviathans down and learns of a facility they are building, only to discover that it is a facility to cure cancer. As one Leviathan states, "We're only here to help". The spirit of Bobby later confirms that while they are curing disease, they are doing it as part of their grand plan to turn humanity into the perfect food source. With the help of Castiel, the brothers learn that the only way to kill the Leviathans is with the "bone of a righteous mortal washed in the three bloods of the fallen" and set out to find the three bloods. Eventually, Dean and Castiel kill Dick but are dragged into Purgatory as a result, while Sam is left alone to deal with Crowley, who plans to rise to power now that the Leviathans are disorganized.

Key plot points presented in the season were Sam struggling with the constant hallucinations of Lucifer and the ghost of Bobby "haunting" Sam and Dean through his alcohol flask. Bobby's rage toward Dick Roman slowly causes him to become avengeful spirit, and Sam and Dean ultimately deal with this by burning the flask at his request, thus destroying Bobby.