Ring of Shadows

From Quake Wiki

This handy little device makes you completely invisible except for your eyes. It's really quite entertaining to fight against people who have this ring, because a pair of gaping eyeballs flying through a room firing rockets is one of the funniest things you'll ever see. It gives you the ability to sneak up on people, but you are pretty easy to spot once they notice you're there, so be cautious. If you have a powerful weapon, you can take them out before they really notice you, so it won't be too much of a problem.

Items in Quake
Health | Ammo Boxes | Mega-Health | Quad Damage | Biosuit | Pentagram of Protection | Ring of Shadows | Green Armor | Yellow Armor | Red Armor