Monstrous Onslaught is a uncommon Sorcery card from Commander Legends (CMR), collector number 244. Current market price: $0.21 (foil $0.17), 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.