Amulet of Spell Immunity

Here's a full write-up for an item mentioned in passing in Supplement IV: God, Demi-Gods & Heroes by Robert Kuntz and James M. Ward.

Amulet of Spell Immunity

An ancient goddess of magic and motherhood whose name is now lost to antiquity first bestowed the method of creating these potent magical defenses upon humanity at the dawn of its first civilization. The secret procedure for the creation of Amulets of Spell Immunity were lost--as was much early magical lore--during the Revolt of the Battle Pyramids. Therefore any Amulets of Spell Immunity discovered today are at least 5,000 years old.

The amulet takes the form of a small round or oval medallion of old gold, slightly larger than the largest coins. Both sides of the amulet are covered with ancient pictograms, now well worn. An off-center hole pierces the medallion, allowing a cord, leather thong, or very fine chain to be threaded through it. A full 50% of all encountered Amulets of Spell Immunity show signs of past use in the form a brittle texture not usually associated with gold and one or more thin cracks along the surface.

When worn around the neck, an Amulet of Spell Immunity grants the wearer immunity to a single spell. The spell is not cancelled; if more than one person can be targeted (such as an area effect attack), the others are still subject to the spell.

Each time the Amulet protects a wearer from a spell, there is a 50% chance that the enchantment wanes and the amulet becomes damaged. A fully intact amulet becomes brittle and cracked, as described above. An already brittle and cracked amulet (whether found in that condition or through repeated use) crumbles into pieces of a dry, spongy material not unlike old cork.

Since these Amulets were created in the earliest days of Magic Use, they only protect from a narrow range of possible spells:

01-08 Charm Person
09-16 Sleep
17-23 ESP
24-30 Hold Person
31-37 Fire Ball
38-44 Lightning Bolt
45-51 Slow
52-57 Polymorph Others
58-62 Confusion
63-66 Charm Monster
67-72 Magic Jar
73-77 Cloudkill
78-83 Feeblemind
84-88 Flesh to Stone
89-92 Death Spell
93-96 Geas
97-00 Disintegrate

The alchemist Grumthrap Viloodle theorizes that the strange residue of a destroyed Amulet of Spell Immunity is not entirely useless. In his work Secrets of Alchemy Revealed Viloodle speculates that this substance can be used as a substitute material component for the spell the amulet previously protected against and/or as the key ingredient in a potion of similar effect.