Widespread Brutality is a rare Sorcery card from War of the Spark Promos (PWAR), collector number 226p. Current market price: $2.60 (foil $7.25), TCGplayer market prices via Scryfall.
Amass Zombies 2, then the Army you amassed deals damage equal to its power to each non-Army creature. (To amass Zombies 2, put two +1/+1 counters on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)