Backlash

With your advanced understanding of the flow of magic, you can now modify incoming damage and store it as elemental energy in your weapon for later use.

As an action, you can cast this spell and you gain 1 charge on your weapon. For the duration of the spell, for every increment of 15 damage taken, you gain 1 additional charge to a maximum of 4. You can expend 1 or more of the charges on a hit to deal 1d6 of damage. The damage dealt can be any 1 type of element.

(fire, lightning, cold, thunder, or acid) In addition, while the spell is active, your weapon is considered magical for the purposes of overcoming magical resistances and you cannot be disarmed.