Wondrous Item, rare (requires attunement)
An amulet magically forged in the depths of a pool of pure energy. The amulet core stores three charges of excess mana from the wearer.
When the wearer casts a spell that normally consumes a spell slot they may use a bonus action to roll a 1d20 + (spell attack modifier - spell level) against their spell save DC.
On a success the amulet refreshes your spell slot. On a failure a charge is consumed from the amulet to refresh the spell slot. The amulet will regain its charges at the end of a long rest. If all charges are spent, the amulet loses all magical properties and ability to hold a charge.
Notes: Jewelry