Dean awoke with a jerk; power rushing through his body and the mark glowing brightly on his arm. It was time, and though he didn't know just how the First Blade had gotten into his hand, he knew exactly what he was meant to do with it. Closing his fingers tightly around the hilt of the jawbone dagger, he rises from the bed; eyes black, and smiling.
"Sammy," he calls out. "Sammy, where are you?" He faces the door, standing just feet from where it would open.
"Dean!" The door flies open, and Sam, breathless from his run to his brother, steps from the lighted hallway into the door frame. "What's wrong," Sam calls out, peering into the darkness of the unlit room.
"Goodbye Sammy," Dean says, the blade arcing upward and jamming itself through Sam's chin and up into his skull. It was just as Cain had said it would be, this was the inevitable end of Dean's journey. Much as Cain had killed his own brother, causing him to become a demon, now Dean, as a demon, had taken the life of the only person in the world that meant anything to him anymore.