IMPINVIS.ITM is an undroppable ring that can be equipped by a non-player character/creature in the game.
The purpose of the item is to ensure the creature is under a permanent Improved Invisibility effect.
This invisibility can't be removed, or dispelled, and is not affected by divination effects such as Invisibility Purge, Oracle, True Sight, or a thief's Detect Illusion ability.