"Is that what you want?" Lucifer demanded, his hand snapping out and grabbing Crowley's jaw. "Being human? Maybe the Winchesters would love you if you were human. Maybe Dean would love you if you were human. Or maybe it doesn't matter. Maybe you'd love to stop, and you can't. Castiel made his blood purer than pure, and you can't get enough of it."
He turned Crowley's head, forcing him to look at the vial in Lucifer's hand — just as he tipped it over and spilled Castiel's blood to the floor.