Sam and Dean discover that the Leviathans are poisoning the human food supply with a corn syrup additive. This additive makes humans lethargic and "tasty" for Leviathans but is lethal to other monsters, such as vampires, who feed on them.
His presence is marked by icy mirrors and flickering lights. [S7E22] There Will Be Blood
The episode ends on a grim cliffhanger that leaves the Winchesters nearly alone: Sam and Dean discover that the Leviathans are