Fulgrim leaned in, rather impressed with himself, and said, "Why, a spell that makes you disappear from sight! While you can still be heard, smelled, tasted, and touched, eyes will fail our enemies."
Considering the follow-up question, Fulgrim shrugged. "Well, we still have that chest to consider in the other room. If we want instead to go deeper, we can go to the east, to the west, or check the secret door located in the middle of the far hallway."