User blog comment:Nightband/Season Seven Plot Hole?/@comment-3045936-20121212173612

Souls are immortal meaning the live on forever, but the physical shell of humans and some none immortal monsters as not ALL monsters stop aging dies. Immortal beings like demons are just spirits hence they are immortal, Angels are ethereal (meaning more heavenly kind of spiritual from a religioish view) are immortal, pagan gods either the spiritual like the vanir, the physical like the majority, or the ones like Atropos that can be either are immortal. But, they are already immortal as they don't have anything to keep their existence if they die. Inshort, deities, angels, leviathans, spirits, ghosts, demons, WHATEVER makes them as an immortal being is like their bodies are their souls. So if they die, they just end like how Castiel describe when Lucifer killed him.

As for the Leviathans, they don't have souls but whatever comprises them as beings is sent back to Purgatory. if they are killed. Hell, their true form on Earth is black liquid. So maybe they are just black liquid that has a mind and can morph at least in Purgatory into creatures.