Galvanic Discharge is a common Instant card from Modern Horizons 3 (MH3), collector number 122. Current market price: $0.24 (foil $0.30), TCGplayer market prices via Scryfall.
Choose target creature or planeswalker. You get {E}{E}{E} (three energy counters), then you may pay any amount of {E}. Galvanic Discharge deals that much damage to that permanent.