Gray Merchant of Asphodel is a common Creature — Zombie card from Theros (THS), collector number 89. Current market price: $1.27 (foil $6.98), TCGplayer market prices via Scryfall.
When this creature enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)