This item comes from the new Legionfall supply cache (earned from getting paragon rep with Armies of Legionfall). The tooltip states it gives 200 Attack Power against demons outdoors in the Broken Isles. The actual buff you get from consuming it states you deal 200% increased damage against demons outdoors in the Broken Isles. This is not widely known yet and not many are available (it's not a guaranteed drop from the supply cache), but it works against a certain imp mother that a certain Rogue spec has to kill for their artifact challenge. I've tested it myself, and it does what it says it's supposed to do. Expect this to be hotfixed to not work in the challenge once Blizzard gets wind of it. Until then, if you can get your hands on one, go get your artifact appearance. It's a 15 minute duration, counts as a flask, and does not persist through death. I've seen it up for 500k-800k gold on my servers, so the secret is getting out. Don't expect this to last for long. There's also no guarantee Blizzard won't roll back the challenge from people who use it (if AP abusers are any indication, they probably won't). I don't think this is an exploit as the item likely does what it's supposed to do. It working in the challenge is probably just an oversight.
Edit: It's been patched.