The Ring of Shadows is very rarely used in custom maps.
If you are playing a map that has the Ring of Shadows, offensive classes should get it. Being invisible makes it much easier for an attacker to run into a base without being fired at. However, enemies will still be told that the flag has been taken and they can see the flag behind the invisible player, so don't use it expecting an easy trip in and out of the base.
- This item is from the original Quake, as the original Team Fortress is based on it, which gives TF access to all of Quake's items.
- This item is likely a reference to J.R.R tolkien's The Lord of The Rings, in which the much sought after ring makes you invisible as it does here.