Amulet of magic resistance
The default randomized appearance associated with it is a rare amulet in SlashTHEM, and an oblong amulet in EvilHack and Hack'EM.
The amulet of magic resistance appears in different places dependent on the variant.
The Sokoban prize amulet has a 1⁄2 of being an amulet of magic resistance.
Giant and tortle aligned priests have a 1⁄21 chance of generating with an amulet of magic resistance, the same as the chance for other aligned priests to generate with a cloak of magic resistance. Giant and tortle wizard player monsters will generate with an amulet of magic resistance, and other player monsters created at level 20 or higher may generate with an amulet of magic resistance.
The Wizard of Yendor has a 1⁄4 chance of generating with an amulet of magic resistance.
Amulets of magic resistance allow for more flexibility in most players' choice of armor, especially for ascension kits - it is of particular importance to giants and tortles, whose choices are limited to either the amulet or a artifact that must be carried or wielded.