"I-" Sam began, but he didn't know how to finish. In contrast to Dean's restless anger, he felt hollow, emptied of everything except for dread and guilt. He had turned Lucifer down, and Cas had taken that on instead, and then Dean had tried to make up for that mistake by offering himself.
It was too much to take in all of a sudden. He wanted to do something, to come up with a plan to get them out of this, but nothing was coming to mind. Well, nothing except-
"He might take me." His voice came out rough, but steadier than he felt. "Lucifer must know he'll burn through Cas's vessel eventually. He'll burn up everyone but me."