"Oh this is going to be so much fun!"
After opening the door to Purgatory, the angel, Castiel gained newer, stronger powers and after killing Raphael (Supernatural), Castiel proclaimed himself to be the new God.
Over the course of many months, Castiel then embarked on a campaign to become a better God but unfortunately, the Leviathans later gained control of his body, declaring that Castiel was "dead" before attacking Dean Winchester and Bobby Singer.
Upon realizing that the vessels were overwhelming him, Castiel headed to a nearby lake and sank into it, the end result causing his vessel to dispel although it released all the Leviathans trapped inside Castiel's body.
Leviathan Castiel is a psychopathic mass-murder who takes sadistic and childish glee from inflicting any psychological wounds on anyone, a key example being when Leviathan Castiel tells Dean that the real Castiel is dead and that they're running the show now.