Apocalypse 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.

Golbez, Crystal Collector

Whenever an artifact you control enters, surveil 1.
At the beginning of your end step, if you control four or more artifacts, return target creature card from your graveyard to your hand. Then if you control eight or more artifacts, each opponent loses life equal to that card's power.