The death and birth of an angel
Takes place between season 4, episode 22: "Lucifer Rising" and season 5, episode 1: "Sympathy for the Devil".
What happened to Castiel after we left him at Chuck's place?
Castiel stood by Chuck's side as the archangel got closer. It must've known he was there, because it brought backup.
Archangels are the most powerful angels of the lord and them coming to get him in a group of three let Castiel understand that they were desperate.
Chuck was shaking where he stood, but he seemed more concerned over Castiel's faith than his own.
"Don't worry." Castiel said and faced the man. "You're going to be fine."
"It's not me I'm worried about." Chuck replied and confirmed Castiel's suspicions.
He closed his eyes as the archangels reached them and the last thing he heard was Chuck's cry of horror as they grabbed Castiel. The impact of their touch blew his vessel into tiny matters of human substance.
For the first time he felt pain, the way he imagined humans felt it and just as he thought he couldn't bare it any longer, everything went dark and he was no more.
A loud cry was increasing in strength and not until it reached its full level did Castiel realize that it was his voice. Not the voice he'd used by the cords of Jimmy, but his true voice.
Flickering lights were surrounding him and constantly grew in size, forming a light so bright it should've blinded him, only it didn't.
Instead he couldn't resist to stare straight into it.
He felt naked, vulnerable, yet incredibly strong at the same time.
All around him was light, warmth and something else. A feeling he had only just begun to understand, a feeling he now felt the full power of and it was overwhelming.
Love.
Just a word to most angels, but to him now something so very real.
Millions of thoughts were spinning around in his head, too many for him to make out a single one.
He was floating, soaring and he lifted his hands to his ears, understanding the agony he had put Dean through in his attempts to talk to him with his true voice.
Suddenly everything fell silent and he slowly removed his hands.
The he heard it, or felt it more like, a voice, a sensation unlike anything he had ever experienced before.
That's when he knew who it was that had saved him, brought him back to life and for the first time ever, Castiel cried.
At first he got frightened by his own reaction, this new emotion, but then he loved the way it felt.
So relieving, so peaceful.
He was given instructions, all along surrounded by a light of love and warmth, embraced in it.
A massive shock wave hit him and with that he was back on earth.
At first he just stood there, trying to take it all in and after gently touching his own face, then looking down on the trench coat he was wearing, he could tell that he'd been given the form of his old vessel.
This was probably so that Sam and Dean would recognize him.
As he thought their names in his head, something like a bolt of lightning hit him, although it didn't hurt.
Instead it made his mind sharp and his mission was clear to him.
He'd been brought back for a purpose - to save Sam and Dean.
God had not sent Zachariah to bring Dean with him by cheap tricks, but he now sent Castiel and he knew that this was an extremely important task he'd been given to do.
It gave him a determination he'd never had before and as the location of the Winchester boys was showing up in his mind, he felt something in the right pocket of his coat.
It was a dagger, same kind Uriel had used to kill their brothers and sisters.
But Uriel had acted on his own will, not God's.
Castiel remembered the vile actions of his former friend and ally, took a firm grip of the dagger and just like that - he was off.