And that would be the cue for resorting to magic. Childermass doesn't move through the shadows, per se, but more like he becomes one that darts flat across the floor, only to allow him to spring up again as suddenly as he'd vanished. To the plain eye, it's almost like teleportation, almost.

It also puts him directly in Steven's path, standing right in the open door, between him and escape.

"He can't hear you, Steven," Childermass continues on sounding terribly calm, like this isn't outright horrible or out of the ordinary whatsoever. "He's not here."