Pyromancer's Ring

Ring, uncommon (requires attunement by a spellcaster)

When you use a spell slot to cast a spell that deals fire damage while wearing this ring and attuned to it, the ring gains a number of charges equal to the level of the spell slot expended to cast the spell. When you deal fire damage while wearing this ring and attuned to it, you can expend all charges stored in the ring to gain a bonus to the damage roll equal to the number of expended charges.

The ring loses all stored charges when you finish a long rest or if you unattune from the item.