As described in http://eressea.army-of-darkness.it/viewtopic.php?t=246 the English description of this item seems to still be missing. I tried the SHOW "ring of invisibility" with unit ag5b of faction suno, timestamped 1120978116308 in the tutorial game.

My suggested translation is: This magical artifact has been used since ancient times by Elves to conceal themselves from their enemies. Other races have also learned the value of these rings after encountering Elves - after all the ring makes its wearer invisible to normal eyes, and only magical methods enable the wearer to be discovered.