Scarab of Protection

From Dungeon Master's Guide, page 199. Found on Magic Item Table I.

Description
Wondrous item, legendary (requires attunement).

If you hold this beetle-shaped medallion in your hand for 1 round, an inscription appears on its surface revealing its magical nature. It provides two benefits while it is on your person:
 * You have advantage on saving throws against spells.
 * The scarab has 12 charges. If you fail a saving throw against a necromancy spell or a harmful effect originating from an undead creature, you can use your reaction to expend 1 charge and turn the failed save into a successful one. The scarab crumbles into powder and is destroyed when its last charge is expended.