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Experience
Castiel had experienced many things in his existence, he had watched as the morning star fell from home. He had witnessed Micheal and Gabriel destroy the cities of Sodom and Gomorrah. He watched as the Son of God rose Lazarus from his tomb. Although Castiel had witnessed these things from Earth he had never been given his own task from God, or his superiors. He was simply taken to events as a fledgling, and the archangels favorite.
Now he had been given he first task by Micheal himself, and a daunting one at that, but he was ready to go to Earth and complete it. Heaven had not been the same since the rebellion, his mentor had fallen and his flight companion had disappeared. Castiel would lay siege to hell in Micheal's place, to rescue the righteous man. The night before their siege Micheal came to Castiel, who was sitting on a bench in his favorite place in heaven. The air shifted and Micheal made his way over to Castiel.
"Are you nervous little one?" Micheal asked taking a seat next to Castiel on the bench. The archangels had taken to calling Castiel little one because when he was created he was small enough to fit in the palms of Lucifer's hands. "Yes." Castiel answered honestly as his wings curled in on himself. He was nervous, being it would be his first time entering hell.
"This will be one of the most important men to walk this Earth little one, not the only one, but a very important one in the years to come. I choose you for a reason little brother" Micheal said as he curled his wings in to comfort Castiel. "You will do just fine little one, why are you so nervous?" Micheal asked shifting his eyes to look over at Castiel. "You are worried that you will run into the morning star, little one you watched as I sealed him into the cage. I assure you, you will complete this task, perhaps better than even I could" Micheal told him shifting his feathers.
With that Castiel spoke, "That is not true and you know it brother, I am merely a child compared to you." He said with a flutter of his wings. "I will not lie to you little one, this siege will not be like others that you have heard stories about, it will be dangerous, and you will not be the same when you return, but as Father commands, it must be you to raise the righteous man, if it was any different I would keep you home and safe." Micheal retorted standing to face Castiel. "Castiel, you will do what is asked of you, and nothing more brother" and with a shift of air Castiel was alone again, and Micheal was gone.
Micheal had never called Castiel by his given name, and that punctuated the seriousness of this task, and Castiel would not let Micheal down. If he was commanded that is what he would do, because Castiel always obeyed and order.
Later, much much later Castiel would look back on this memory fondly, as a turning point in his entire existence. Only God knew why Castiel was chosen to raise Dean Winchester the righteous man from hell and Castiel no longer questioned it. For God had given Castiel everything he could ever want in existence. Castiel had seen many things in life, but the memory that would stay with him forever, was when he flew through hell and caught his first glimpse of the most beautiful thing he had ever seen, a soul, alight like a beacon, the soul of Dean Winchester.