On this week's episode of Supernatural…
Castiel's vessel proves too weak for the Leviathan and they leave it and travel into the water system as a black ooze. Castiel is dead.
The Leviathan begin to inhabit humans, as well as feast on them. A number of them have set up a base at Sioux Falls General Hospital.
Sheriff Jody Mills happens to be a patient at the hospital and notifies Bobby about the weird creatures taking over.
Bobby learns that silver doesn't affect the Leviathan.
Meanwhile, Sam's hallucinations worsen as he continues to have trouble recognizing reality. Dean is able to prove to Sam through pain, that Lucifer isn't real.
Sam and Dean return to Bobby's to find the place destroyed and Bobby missing. The two encounter a Leviathan. They crush it with a car, after Dean's leg is broken and Sam is hit in the head.
Dean calls an ambulance, but unfortunately it is taking them back to Sioux Falls General Hospital where the Leviathan reside.