As Sam stayed behind to draw the Rabids away, using himself as bait to find a cure for the infection spreading through his own veins, Dean escorted Jenna and the infant to safety.
Sam didn't answer immediately. He was staring at the road ahead, where the asphalt seemed to shimmer with heat that shouldn't be there. "We let it out, Dean. We actually let it out." [S11E1] Out of the Darkness, Into the Fire
If you want to explore more about this specific era of the show: The psychological bond between Sam’s secret struggle with the Rabid infection Castiel’s torture at the hands of the angels As Sam stayed behind to draw the Rabids
The sky above the Impala didn't just go dark; it bruised, a deep, oily purple that bled into the horizon like ink in water. Sam and Dean Winchester stood by the hood, the ringing in their ears finally fading, replaced by a silence so heavy it felt physical. They had just released the Darkness—an ancient, primordial force—and the world didn't end with a bang. It ended with a breath. "We let it out, Dean
"It’s a localized infection," Sam whispered, hopping out of the car to inspect a fallen road worker. "Whatever that smoke was, it changed them."