Not a cursed item per se, this is an old magical device designed to allow rangers and woodland spies to communicate visually over long distances. Used as a regular spyglass, however, it can bring trouble to ignorant adventurers.
This appears as a small telescope of arcane construction. It is brass, and covered in dim runes. It will radiate faint magic, if tested. While investigating the spyglass, it will be learned that it works just like a regular one, except that it can see any distance out to the horizon, as if whatever is being viewed were only fifty feet away or so.
When first used when it really matters, any animal or intelligent creature that is viewed through this glass will somehow sense they are being watched, and will even tend to turn and look directly at the user, even if they are many miles away! Wild animals will generally become alarmed and run off. Intelligent beings will apparently even be able to see the user of the spyglass as if they were only fifty feet away. When the user puts down the spyglass, the effect on the subject of the viewing stops, though they'll know someone was watching, as well as what they look like, and approximately how far away they are.
Image: Spyglass by Hydrargyrum, released under a Creative Commons Attribution-Share Alike 3.0 Unported License, modified in Inkscape by David