The news erupts all over Heaven the moment it happens. Michael's vessel has gone to Hell. It's common knowledge among angels. Castiel hears about it too, and he wonders what the archangel's going to do about the situation. Nothing like this has ever happened before; the closest was when Lucifer's vessel, Sam Winchester, died, and Michael didn't do anything about it, because he wasn't meant to, and it would have meant that his brother could not fight him. It wouldn't have mattered anyway; Dean Winchester interfered, the love between the two brothers pushing them from extreme to extreme in a bid to make sure neither of them were left alone in the cruel and unforgiving world around them. They knew Dean was going to Hell, but they didn't interfere; destiny would make everything as it should be.
Until Dean wasn't rescued, and the first seal was broken, and preparations had to be made for the inevitable fight between the two oldest archangels. As it is on Heaven, so it shall be on Earth.
Castiel doesn't expect Michael to pay attention to him, despite his rank. He's gained a reputation in Heaven, stubborn and unyielding, willing to do anything he's asked.
He's on Earth when he's called up to Heaven by Michael. He doesn't know what the call is about, what the archangel wants to see him for, but he flies up to the Hall of Angels.
When he enters, Michael's alone. He marches to the edge of the throne where the eldest archangel sits, looking up at his older brother, whose head is tilted to the side, regarding the younger angel with a look of curiosity.
'By now, you'll have heard the news about my vessel.' He continues before Castiel has had a chance to acknowledge the information. 'I need you fetch him for me.'
'Why me?' Castiel asks.
'Because God commanded it.'
'Has anyone else tried?'
Michael's expression is blank, unchanging. 'Yes, many. But none of them have succeeded.'
'What makes you think I will be any different?'
'You have a reputation.'
Castiel nods. 'Alright.' He's not one to deny orders, especially not from God.
Michael nods him out of the room, and Castiel leaves immediately, flying towards Earth.
The wind pulls at his wings and he begins falling, plummeting towards the earth at a rapid rate. Lights from distant cities far below him grow brighter and brighter as he nears them, and he can hear the rumble of street cars, and if he doesn't regulate his speed soon, he'll crash. Crashing would leave him wounded, and he needs all his strength to pull the Righteous Man from his prison in Hell.
He's less than ten feet from the ground as he rights himself, the wind catching behind his wings again so he regains balance, and he can focus on opening a gate to Hell, channelling his grace, allowing his every molecule to become one with those of the earth, spreading himself as microscopic vibrations within molecules, transferring himself through the earth and into Hell.