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 possessor. The figures appear as "shadows" but become clearer and more visible as the enemies get closer to the possessor.
Barty Crouch Jr, while impersonating Alastor Moody, had Moody's Foe-Glass in his office during Harry Potter's fourth year at Hogwarts School of Witchcraft and Wizardry. He said he's not really "in trouble" until he can see "the whites of their eyes." Later in the school year, after Cedric Diggory's tragic death, it was revealed that Crouch's enemies shown in the glass were Albus Dumbledore, Minerva McGonagall, and Severus Snape, all notable members of the Order of the Phoenix.
When Harry and the other members of Dumbledore's Army first entered the Room of Requirement, he found, among other Dark Detectors, the fake Moody's Foe-Glass. By this time it was cracked, but still functional.
- Harry Potter and the Goblet of Fire (First appearance)
- Harry Potter and the Goblet of Fire (film)
- Harry Potter and the Order of the Phoenix