Foe Glass

From Hpwiki

A Foe Glass is a magical instrument similar to a mirror to detect foes. They appear as shadows on the glass, with the shapes getting more distinct the closer the "foe" comes. Apparently the definition of foe depends on the person owning or using the glass, as among others Albus Dumbledore appeared in Barty Crouch, Jr.'s foe glass. This glass might have really belonged to Alastor Moody but that's not clear.