Harnessed Lightning is a uncommon Instant card from Kaladesh (KLD), collector number 117. Current market price: $0.30 (foil $0.27), TCGplayer market prices via Scryfall.
Choose target creature. You get {E}{E}{E} (three energy counters), then you may pay any amount of {E}. Harnessed Lightning deals that much damage to that creature.