Apocalypse Demon is a rare Creature — Demon card from Hour of Devastation (HOU), collector number 58. Current market price: $0.25 (foil $0.95), TCGplayer market prices via Scryfall.
Flying Apocalypse Demon's power and toughness are each equal to the number of cards in your graveyard. At the beginning of your upkeep, tap this creature unless you sacrifice another creature.