The Empty

"Cosmic Entity: Before God and Amara, creation, destruction, Heaven, Hell, your precious little Earth, what was there? Castiel: Nothing. Cosmic Entity: Yes. That's right. Nothing. Nothing but empty and you are soaking in it. Angels and demons, you all come here when you die (...) sleeping an endless, peaceful sleep."

- Cosmic Entity and Castiel

The Empty is a void that existed before God or the Darkness. It is stated that nothing has ever escaped the Empty and it is where angels and demons go upon death.

Background
According to the Cosmic Entity that controls the Empty, the realm existed before anything else, including God and the Darkness. As soon as they die, every angel and demon goes to the Empty where they sleep eternally. As long as the inhabitants of the Empty sleep, so does the Empty.

Season 11
In Form and Void, hunter Sam Winchester meets the Reaper Billie who tells him about the Empty. Billie states that when the Winchesters die, she will throw them into the Empty so that they can never be resurrected again. Billie notes that Sam will soon go to the Empty due to his Rabid infection. In Red Meat, Dean commits suicide via drug overdose to negotiate with Billie to bring Sam back. Billie refuses, stating that she would never make a deal with Dean even if Sam wasn't dead. "The Empty awaits" Billie announces as she prepares to take Dean to it. However Dr. Kessler manages to revive Dean and he avoids being sent to the Empty.

Season 13
In Patience, Jack calls out to Castiel, awakening him where he slumbers inside the Empty.

In The Big Empty, Castiel wanders the void, calling out for anyone. He is greeted by the Cosmic Entity who takes on Castiel's form to communicate with him. The Entity explains to Castiel what the Empty truly is and is shocked that Castiel is awake as not even God has power over the Empty. The Entity is enraged that Castiel is awake because as long as Castiel is awake, the Entity can't sleep. The Entity painfully goes through Castiel's memories and tries to convince him that he is worthless and should go back to sleep. Castiel refuses and demands that the Entity send him back to Earth, threatening that he will fight the Entity for an eternity if it doesn't release him. Castiel's efforts are ultimately successful and the Entity returns him to life on Earth in order to be able to return to its slumber.

Known Inhabitants

 * Cosmic Entity
 * Every currently deceased angel and demon

Former Inhabitants

 * Castiel

Appearances

 * Season 11
 * Form and Void (mentioned only)
 * Red Meat (mentioned only)
 * Season 13
 * Patience
 * The Big Empty