The Shadow

The Cosmic Entity is the oldest being in existence, having been around before even Amara and God. It is an extremely ancient and powerful Primordial being that supremely controls The Empty.

Background
The entity controls the Empty and has existed before God or the Darkness. When angels and demons die, they go to the Empty where they eternally slumber, allowing the entity to do the same. If even one of its charges is awake, the entity is awoken as well until the awakened being is asleep again or out of the Empty.

Season 13
In The Big Empty, after the Seraphim Castiel is awakened by Jack's telepathic call, the entity is awakened again. It arises in the form of a pool of 'water' inside the Empty after Castiel walks by before taking on a humanoid but still watery form.

As Castiel makes his way through the Empty, he senses the entity and calls out to it. The entity takes on Castiel's form to communicate with him directly, though it speaks with a distinct voice. The entity identifies itself as "just your friendly, neighborhood cosmic entity" and states that it has taken on Castiel's form as its real form is too frightening. The entity explains to Castiel that the Empty is what existed before everything, including God and the Darkness and Castiel is "soaking in it."

The entity tells Castiel about how every angel or demon that has ever died is sleeping an eternal sleep inside the Empty and is confused by how Castiel is awake as "in all of forever", nothing has ever awakened inside the Empty. Castiel suggests that the Winchesters must've made a deal, but the entity states that only it has power over the Empty with not even God having power there. The entity puts its hand to Castiel's forehead and painfully goes through his memories in an attempt to find out what could've woken up Castiel.

Afterwards, the entity explains to Castiel, who remains in pain, what he did and that it wants Castiel back to sleep. Castiel realizes from the entity's description that having Castiel awake is painful for him which the entity confirms, stating that if Castiel can't sleep, it can't sleep. The entity insists that it likes and needs sleep, causing Castiel to suggest that it get rid of him by sending Castiel back to Earth. The entity suggests that it instead throw Castiel so deep into the Empty that he can't bother it anymore, but Castiel recognizes that the entity is bluffing and that it won't work as the entity would've already done it if it would work. The entity acknowledges that Castiel is right, but refuses to send him back, telling Castiel that "its not part of the deal."

The entity states that it has gone through all of Castiel's mind and knows everything there is to know about him and claims that there is nothing back on Earth for Castiel. The entity grasps Castiel's head and painfully shows Castiel visions of his grace being stolen by Metatron, his death at the hands of Lucifer, the Escape of the Leviathans and Castiel returning the souls to Purgatory. The entity beats Castiel, attempting to convince Castiel to return to sleep. However, Castiel refuses the entity's demands and declares that no matter what the entity does, he will fight it for eternity if he is not released. The entity angrily denies Castiel's words as Castiel continues to demand his release.

Castiel later awakens in a meadow on Earth and realizes that the entity has resurrected him as Castiel demanded.

Powers and Abilities
Being the oldest being in existence, the entity possesses immesurable supernatural power, allowing it to do almost anything it wishes within its own realm.
 * Nigh-Omnipotence (within the Empty) - Being the oldest being in existence, the entity has the power to do nearly anything and has complete control over the Empty. The entity claims that not even God can overpower its influence on the Empty and all within it. However, the entity lacks the ability to force something that has awakened in the Empty back to sleep and must either convince them to return to sleep on their own or expel them.
 * Shapeshifting - Due to its true form being too terrifying for anything's eyes, the entity can take on the form of the being it is speaking to, though it speaks with a distinct voice instead of mimicking the being's voice.
 * Super Strength - The entity was able to effortlessly beat up Castiel.
 * Memory Manipulation - In order to learn more about Castiel, the entity painfully dug through his memories. The entity later used this power to show him some of his most painful memories in an effort to convince Castiel that he was worthless and should go back to sleep.
 * Resurrection - The entity was able to resurrect Castiel, returning him to Earth in the process.
 * Reality Warping - Despite Castiel's vessel being burned, the entity was able to restore it with no sign of damage.

Appearances

 * Season 13
 * The Big Empty