William was already covered up with that fog. And in their eyes, he was suffering from a slowdown effect from it. Plus, his vision started to get affected, and gradually he started to see nothing at all of the outside world.
Darkness might be considered their strongest ally, but at this moment it turned to be their deadly foe. The dark fog blocked their vision totally, and also senses. So they didn't get what was happening to William, and only could imagine it.
And what they imagined was wrong! William simply let his knives rotate around him when he got engulfed with all the black fog, to prevent a single speck of it from falling down on him.
And so he got rid of all the negative effects he suffered, restoring his fast speed and reflexes. As for his vision, it didn't enhance anything as the fog was pitch black, masking anything and everything outside.
