Signature Slam is a uncommon Instant card from Modern Horizons 3 (MH3), collector number 168. Current market price: $0.27 (foil $0.28), TCGplayer market prices via Scryfall.
Put a +1/+1 counter on target creature you control, then each modified creature you control deals damage equal to its power to target creature you don't control. (Equipment, Auras you control, and counters are modifications.)