Dawn of the Monsters

Dawn of the Monsters can be your commander.
At the beginning of your upkeep, if there are less
than 20 cards in your graveyard, discard a card,
mill a card, and sacrifice a permanent.
When Dawn of the Monsters enters the
battlefield and at the beginning of your upkeep,
if there are more than 20 cards in your graveyard, create Cthylla, a 10/10 red, green, and blue
Legendary Horror creature token with flying, hexproof, menace, and trample. Then exile
Dawn of the Monsters.