Shellshock is a rare Instant card from Teenage Mutant Ninja Turtles Eternal (TMC), collector number 25. Current market price: $0.28 (foil $0.25), TCGplayer market prices via Scryfall.
For each opponent, choose up to one target creature that player controls. Shellshock deals X damage to each of those creatures. You create a Mutagen token for each creature dealt damage this way. (They're artifacts with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")