Lucifer was growing weary of repeating himself, but he did anyway, "I'm the only thing keeping him from slipping through the bars. This is a cage for angels, not vessels."
"But even if he did get out, he'd just be torn apart." Michael said for the hundredth time.
"You forget brother, Hell is my kingdom. They can't hurt him, not anymore. But he can hurt them." Lucifer took one last, long, languorous stretch with Sam's body and then left his vessel.
Sam fell through the Cage and landed in the center of Hell.
Hell was a prison for souls, but it was also - out of necessity- a place of flesh and blood. Tangibility was a prerequisite for so many forms of torture.
Sam woke mad with hunger. His senses were overwhelmed by the scent of sulfur and blood and it was all around him.
It was a fairly young demon that came across him first. It stopped dead in its tracks when it saw Sam getting slowly to his feet. Young as it was, the demon had heard of Sam Winchester. It knew he was Lucifer's vessel, but it could tell that Lucifer was not in Sam currently. It also knew that Sam Winchester was extremely dangerous to demons who were possessing people on Earth. He could consume them, it was said. Demon blood made him powerful, let him send demons back to Hell, or tear them to shreds, or wipe them from existence. But they were in Hell. It had no host body, it was safe!
Sam Winchester stood to his full height, looked at the demon, and said, "You're bleeding."
There was no safety in Hell. Hell made flesh when flesh was needed.
"How long have we waited, Lucifer?" Michael asked, annoyed. " Face it, your vessel has been claimed by Hell, he isn't coming back."
Lucifer smiled, "I can see what he sees, you know...and it's...glorious. He'll be back, he's just not quite done yet."
"You're mad." Michael scoffed.
"Do you know how many souls are in Hell, Michael? How many demons I've made?" Lucifer closed his eyes again and watched Sam, watched his perfect vessel, harvest everything Hell had to offer him.
"I don't understand how this is supposed to help us get out of here, brother. Even if your vessel is still alive, how does that help us?" Michael sounded bored again.
"I may as well tell you my little secret, it's not like you can stop it, not now." Lucifer got up and sat down next to Michael, "Sam was made to contain me. All of me."
Michael raised an eyebrow in mock surprise.
"Do you know how I turned all these billions of souls into demons, Michael? I started with a tiny, tiny fragment of my grace. I seeded Lilith with it...and in her it grew and flourished until she was a power in her own right. Every demon, every single one, has a fragment of me in them. That's what Sam is reclaiming. By the time he's sated, he'll have all the power of Hell inside of him - all of it. He'll spring the Cage open with a thought." Lucifer saw Michael's light pale as his declaration sunk in.
And then Sam returned. Sam walked through the Cage walls like they were never there, and the angels' prison turned to ash around them.
Lucifer stood and walked towards Sam, "Sam, welcome back."
Sam looked at Lucifer and said, "You're bleeding."