Apocalypse Demon 
Creature — Demon
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.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL