This appears as a gold or silver ring, sometimes with expensive gemstones. It will radiate magic. This ring will seem to be a Ring of Invisibility, and all initial tests will seem to confirm that.
Upon first being used when it really matters, the wearer will not go invisible, but every other person or creature will, at least from the wearer's point-of-view. From this point on, the wearer will be unable to see any other people, monsters, or animals. The wider world, ordinary plants, furniture, buildings, etc., all these things are still visible, and they can hear people just like always. Attacking these people will not negate this effect.
The wearer can use a Detect Invisible spell of some sort to see other people, for as long as it lasts, but then they go back to being unseen. As far as anyone else is concerned, the victim is still quite visible.
The ring will not come off, of course, nor will the effect end on its own. A Dispel Magic or Remove Curse will allow the ring to be removed, ending the effect. Severing the finger will work, as well.
Optional: This ring does allow the wearer to go invisible, but it won't wear off, and the other negative effects still persist; the character cannot be seen, nor can they see anyone else.
Carved silver ring by Mauro Cateb under a Creative Commons Attribution-Share Alike 3.0 Unported License Inkscape by David