Sam didn't even need to say it, that much was clear by the bright flash of light engulfing both brothers in something that they knew was over their heads. Dean the beginning of it, Sam the big finish, the apocalypse had begun.
"Sammy we need to go, now!" Dean snapped, suddenly becoming blind, the light so bright that neither of them could see anything anymore. A cold wind swept through the room, so fierce and chilling that it knocked both Sam and Dean off their feet and away from the hold that they'd had on each other's jackets.
An earsplitting rumble, one last flash of light, and what felt like an earthquake later, and then all was silent...
Dean hadn't meant to let go of his brother, determined to keep him grounded this one last time, at least until the end which seemed to be a lot sooner than any of them had anticipated. Like maybe now, for instance. But he was no match for the great force that knocked him off his feet, knocked the air out of his lungs.
Hearing that low growl, almost like the earth was opening up below him, Dean felt a fear settle into his gut. There was a force that was coming, Dean could feel it. His brother had done it, unintentionally of course, but he had actually unleashed hell on earth. He knew it well, and this was what pure, unadulterated evil felt like.
Suddenly, Dean felt that same evil rip through him and he fought it, fought against it as hard as he could, but he knew that there was nothing he could do. Where were the angels when this was happening, where was that divine intervention? An evil, greater than any he'd ever known, Azazel, Alistair, none of them compared, made its way into the very depth of the one who was supposed to be Heaven's hope. The last words he spoke as Dean Winchester-
"Sammy, get the hell out of here!"