Blight-Breath Catoblepas is a common Creature — Beast card from Theros Beyond Death (THB), collector number 86. Current market price: $0.09 (foil $0.21), TCGplayer market prices via Scryfall.
When this creature enters, target creature an opponent controls gets -X/-X until end of turn, where X is your devotion to black. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)