[To say he has prior experience is putting it lightly, but that's a detail he doesn't quite remember yet. It will certainly explain a lot when he recalls that he was one.]

They're skilled with illusions, and can draw a person's fears out into visions without even knowing what they are. I remember one pulled people into something like a waking nightmare.

[... so, you know. Seems relevant with the whole "dreamwalking" business.]