Lightning Runner is a mythic Creature — Human Warrior card from Aether Revolt (AER), collector number 90. Current market price: $0.29 (foil $2.79), TCGplayer market prices via Scryfall.
Double strike, haste Whenever this creature attacks, you get {E}{E} (two energy counters), then you may pay eight {E}. If you pay, untap all creatures you control, and after this phase, there is an additional combat phase.