D: Sigil

You bury teeth around the area you want to curse, needing 1 tooth per square feet you want cursed. Creatures that spend at least 1 full hour inside the area become completely blind to anything blue. Blue things aren’t invisible, but those cursed will never notice them by sight alone. The effect lasts as long as the creatures stay inside, and [dice] days after they leave.

Blue + Blind

Adapted from Michael Prescott’s adventure the Sky-Blind Spire