After absorbing souls from Purgatory and declaring himself God, Castiel begins to punish all the wrongdoers in the world. 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 and inhabiting the bodies of many different people around the world. Sam and Dean learn that their weakness is Sodium Borate (also known as 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. The spirit of Bobby later confirms that while they are curing disease, they are doing it as part of their plan to turn humanity into the perfect food source. With Castiel and Kevin's help, 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. Kevin can read the word of God and helps Sam and Dean decipher it to stop the Leviathans. 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.