Foe-Glass

a Foe-Glass is a type of Dark Detector. It looks like a mirror, but instead of a reflection it shows the enemies of its owner. The figures appear as "shadows" but become clearer and more visible as the enemies get closer to the owner.

"Professor Moody" had a Foe-Glass in his office during Harry's fourth year at Hogwarts. He said he's not really "in trouble" until he can see "the whites of their eyes".