Monstrous Onslaught is a uncommon Sorcery card from Aether Revolt (AER), collector number 116. Current market price: $0.10 (foil $0.69), TCGplayer market prices via Scryfall.
Monstrous Onslaught deals X damage divided as you choose among any number of target creatures, where X is the greatest power among creatures you control as you cast this spell.