"Oh that's my Foe-Glass. See them out there, skulking around? I'm not really in trouble until I see the whites of their eyes. That's when I open my trunk."
-- Fake "Mad Eye" Moody / Barty Crouch Jr. (GF20)
This device looks like a mirror, but it does not reflect the scene in front of it. Instead, it shows the enemies – the foes – of its owner. The foes are shadowy, indistinct, out-of-focus figures if distant; the images become more distinct as the foes get closer (GF20).
- The fake Moody kept a Foe-Glass among the various Dark Detectors in his office at Hogwarts (GF20).
- When Dumbledore, McGonagall, and Snape rescued Harry from the fake Moody after the third task, it showed their images (GF35). From that incident, we observe the following.
- The Foe-Glass is tuned to show the foes of its owner, not just those of the person standing in front of it (unlike, say, the Mirror of Erised, which works for whoever stands before it). It showed the fake Moody's enemies when he wasn't watching it, and even when he was unconscious the images remained when Harry and Snape looked into it.
- The Foe-Glass operates without its owner needing to concentrate on it; the fake Moody had forgotten it completely and wasn't watching it, but it still showed his foes approaching.
- During the first meeting of the D.A., a large, cracked Foe-Glass hung in the Room of Requirement that Harry felt sure was the same one that the fake Moody had had the previous year (OP18).
The name of this device is a nice play on words. While a "foe" is an enemy, it is also a homonym for the French word "faux", meaning "false" - a fitting term for a looking-glass that isn't a true mirror.