Castiel couldn't believe that after all this time, he could finally forgive himself for his past. He's finally gotten over everything's that happened.
Sam, Dean, and him fought together. Tean Free Will against the world. The three of them managed to stop God from going fully evil. They convinced him to see that there could be interesting stories without homicide.
The three of them reunited with Jack. Jack with his soul intact. He's whole again, pure and untainted. The four of them are together, as a family.
Castiel has suffered for years, unable to reconcile with himself. But he's finally found forgiveness for himself. He learned to heal himself and taught Jack that he shouldn't do what Castiel did. That he shouldn't dwell on mistakes, but instead focus on making the world better.
The world is safe. The Winchesters aren't facing any threats. All the world threats seemed to have ended. Now they only have to deal with monsters that are nothing compared to the other beings they've faced.
Jack is still learning how to live in the world after being traumatized by his time being soulless and his time in The Empty. Castiel and the Winchesters help the Nephilim with his inner demons. They try to lessen the pains of the scars that they all could still feel. They fix the broken bonds between each of them.
Team Free Will 2.0 is together again. This seems like a miracle. But unfortunately, all good things eventually come to an end. Castiel knew his deal with The Empty was bound to catch up with him eventually.
Castiel steps out the bunker for fresh air. He allows himself to relish in the freedom of just being content, filled with only good feelings. His being is without any of the stress and guilt he used to be full with. Then he feels it. The dark feeling he's only felt in one place. He feels The Empty.
The Shadow takes the form of him. It stands in front of him, grinning. "So, how was your taste of true happiness?" It asks.
"I thought it was impossible, for me to be truly happy," Castiel solemnly replies. And now that he's felt it, he wants to stay there forever.
"I presume that is why you were so confident to make that deal. But, now it's time for you to leave." The Shadow says.
A flare of panic crosses Castiel's eyes. Although he thought leaving would be easy, now that the time has come, he doesn't want to leave his family behind. "Please, let me say a proper goodbye to them, before I go."
"No, it's better for me if they are not aware of why you disappeared."
Castiel feels himself dragged onto the ground. He's pulled away from the world, away from feeling alive. He's on the ground, unable to stand up. He just feels empty, like there's a void inside of him. He fights to stay awake, but his consciousness begins to leave him.
"It's time for you to enter an endless sleep. You won't wake me up this time."
- - - - - - -
The next day, the Winchesters are unaware of whereabouts of their angel friend. Jack is the only one that knowswhere Castiel has gone and grieves the loss of one of his father figures. He eventually tells the Winchesters, who are also consumed by sadness.