Dean passed out due to the amount of energy that he used in his whole body in that final attack. He utilized his "Willpower" in such a way that he felt like his body was a balloon about to explode when filled with too much water.
For a moment, he thought that he shattered his whole body while doing such a thing, but that was just the effect that the technique used to develop his density had. It always made it look like his "Willpower" would explode him inside out due to its intensity. Also, as its shape was denser now, not smooth and light like before, that feeling got worse.
Anyway, it was a relief that he didn't break himself while trying to defeat that Dread.
The boy fainted for just a while, which was fortunate for him since he was still underwater.
For a long while, all he saw were flashes of himself trying to swim to the surface of that lagoon. His mind was turning on and off repeatedly, making his body act on its own to get him out of the water.
