FSSPIR.ITM

FSSPIR.ITM is an undroppable item equipped by a few creatures that are meant to be invulnerable to damage, spells, death and similar events/actions. 
This item is similar to the INVULNER.ITM in function and complement each other.
This item provides immunities to spells of many types, damage types, status effects, and also applies always on "cures" to some status effects as well. 
The wiki isn't listing these as it has no value to the reader or player to dive into the minutiae.
In conclusion, this item, along with others, is meant to safeguard the creature from overt and accidental destruction so that the game plot can be presented without glitching the player's game.