Lucifer didn't fit inside Castiel's vessel. He was too big, he fit in all the wrong ways, a hurricane trapped in a bottle. Castiel used to be that way, but he'd grown into his vessel, to the point where it felt like it was genuinely his body. Lucifer was crammed in, tearing it apart from the inside. Signs of wear were beginning to show: a roughness in his skin, dry patches around his eyes.
He looked down at himself, at his hands. His knuckles were showing wear, too, rubbed raw and red. "Like I told the Winchesters, this wasn't the original deal. This was the backup plan after Sam wussed out."
He glanced off into the distance, then beckoned a little with his fingers as he wandered off, away from Gabriel's post. "They wanted me to fight the Darkness," he said. "I said I could beat her."
Alone. He'd certainly overstated his own skills, but could he be blamed, really? He didn't want to be trapped anymore. Freedom was worth a little lie or two.