Leviathans

There are thing much older than souls in Purgatory. Long before God created Angels and man he created the first beasts: Leviathans. I personally found them entertaining but he was concerned they'd chomp the entire Petri dish. So he locked them away. Why do you think he created Purgatory? To keep those clever, poisonous things out. – Death, 7.01 Meet The New Boss

History

Leviathans, also called the Old Ones, are beasts that pre-date the creation of humans, monsters, Eve and angels, as well as the soul itself. They are therefore among the first creations of God, if not the original. Death describes them as immensely powerful and hungry. Before God went on to create more creatures, however, He felt that the Leviathans threatened to consume and destroy everything 'in God's petri dish'. For this reason, Death states that God created Purgatory and sealed the Leviathans inside to contain them and their hunger.

When Castiel opened the door to Purgatory and took in the power of the souls stored there, he also took in the Leviathans. With the help of Sam, Dean and Bobby, Castiel finally expels the souls back to Purgatory, but some of the Leviathans "held on". Castiel's Vessel is taken over by the Leviathans, and claim that Castiel is dead and no longer in control.

Episodes

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Leviathans manifest

7.01 Meet The New Boss

Whilst ingesting all the souls from Purgatory to became the new God, Castiel also had to take in the Leviathans. The Leviathans struggled for control, briefly taking control of Castiel's Vessel to smite people that even the now-wrathful Castiel didn't intend to, and causing his vessel to physically deteriorate and potentially explode.

With the assistance of Dean, Sam and Bobby, Castiel returns the souls to Purgatory. But "some" of the Leviathans hold on behind, and assumed control of Castiel's vessel. Stating Castiel as dead, the Leviathans attack Dean and Bobby with manic delight, remarking: "Oh this is going to be so much fun."

Leviathans in Lore

• In the Judeo-Christian tradition a Leviathan is a giant sea monster, described in the Book of Job Source
• The Talmud Baba Batra 75b states that the archangels Michael and Gabriel will be the ones who slay the Leviathan. Other legends say God will slay the beast. Source: Leviathan in Judaism
• Death also refers to the Leviathans as "The Old Ones" a term used by H.P. Lovecraft in the Cthulhu mythos to describe ancient powerful alien beings who come to Earth. A race called the Old Ones also features in Stephen King's Dark Tower Series.

References

• Sera Gamble and Bob Singer talk about Leviathans with AOLTV