The Empty
Cosmic Entity: You see, before God and Amara, Creation, Destruction, Heaven, Hell, your precious little Earth, what was there?
Castiel: Nothing.
Cosmic Entity: That's right. Nothing. Nothing but Empty. And you are soaking in it. Angels and Demons. You all come here when you die.
The Empty is first referenced by the reaper Billie, when she is delivering a message to Sam regarding his and Dean's place after the death of Death. She states that nothing is able to come back from it, which is eventually disproven when a Cosmic Entity residing in the Empty revives Castiel and sends him back to Earth.
Before the Universe and even God and the Darkness, there was only the Empty, an infinite black void. The Empty appears to exist outside of creation, and is the final resting place of angels and demons, who slumber peacefully in the vast nothingness. A Cosmic Entity appears to preside over the Empty, sleeping alongside the angels and demons. The rare act of waking up inside the Empty will also jostle the Cosmic Entity from his slumber, forcing him to remain awake for as long as the the angel or demon remains conscious.
Episodes
13.03 Patience
As Castiel's body lays motionless in a black void, he is jostled to consciousness upon hearing Jack whisper his name. Castiel picks himself up in a confused haze and looks out upon the infinite emptiness he has found himself in.
13.04 The Big Empty
As Castiel wanders around the Empty looking for anyone, he inadvertently awakens the Cosmic Entity from its slumber. The Entitiy takes on Castiel's form and speaks in a eloquent voice, telling Castiel that his true form would drive him mad, and explains the purpose of the Empty to Castiel. Revealing that the Empty is the final resting place of all angels and demons, which are unable to awaken from their eternal slumber. Prompting the Entity to question why Castiel is awake, revealing he cannot go back to sleep until Castiel does. The Entity tries to convince Castiel to go back to sleep rather than return to Earth and face everything he's done. However Castiel, steadfastly keeps his resolve and threatens to keep them awake for all eternity driving them both insane until he sends him back to Earth. Realizing his threats of throwing Castiel deeper into the Empty and his beatings will not work on the angel. The Cosmic Entity appears to accept the deal and sends Castiel back to Earth.
