Mage-Biter Ring, A System-Agnostic Magic Item

Mage-Biter Ring

Description:  Golden ring in the form of a stylized serpent swallowing its own tail.  Radiates a strong aura of anti-magic that registers as a chilling void to magical senses.  Powerful investigative spells might detect the information under complications below but require a very difficult check to do so due to the dampening effects of the ring's aura distorting such magic.  

Powers:  The wearer may touch or strike any spell-casting creature, either as an attack action or in reaction to the target attempting to cast within arm's reach.  This does no damage but immediately drains the highest-level spell slot available to the target (when used as an attack) or counters the spell being cast (as a reaction strike).  Creatures with natural spell abilities rather than spell slots lose one use of their most potent ability (or whatever they were attempting to employ).  You'll have to do some rules hacking for games that use power points or other system but the basic effect should always equate to depriving the target of one casting of their most powerful spell that day, or negating whatever magic they were attempting when using the ring as a reaction.  

Possible Origins:  The fine craftsmanship of the ring suggests a Dwarven origin, but the formula for enchanting these items is known beyond their realms.  They are most often made by those who have earned the ire of powerful wizards or priests, often by their own professional rivals.  Many spell-casters shy away from wearing a mage-biter ring for reasons of pride.  Relying on such an item suggests you lack confidence in the superiority of your own magics.      

Complications:  A mage-biter ring can safely absorb seven spells per 24 hour period regardless of power.  Using it more often still counters the spell involved but can catastrophically overload the item with a cumulative 10% chance of an explosion occurring, eg 10% for the eighth spell, 20% for the ninth, 30% for the tenth, etc.  An explosion destroys the ring, costs the wearer a finger, and does Xd6 force damage where X = the number of uses today (including the one that triggered the blast).  The explosion has a 30' radius and creatures other than the wearer receive a save for half damage using the same mechanics your system uses to avoid magical area of effect attacks (in D&D, either a save vs spell or a reflex save depending on edition).  

Design Commentary:  This one's pretty powerful depending on circumstances, but the difficulty of detecting the ring's safe usage limitation makes it quite dangerous to rely on too heavily.