Energy Storage

For the duration, when a spell or cantrip is cast on a target within 30 feet of you, you can use your reaction to absorb a part of the energy of that spell.  When you do so, you gain a number of energy charges equal to the level of the spell you absorbed the energy from (minimum 1 charge).  You can't have more than 10 charges at a time.

As a bonus action when you deal damage to a creature with a spell or cantrip, you can expend any number of energy charges you stored.  When you do so, the target takes a number of extra lightning damage that depends on the number of charges you expended:

1 Charge: 1d4

3 Charges: 5d4

5 Charges: 10d4