As Sewer Nemesis enters, choose a player.
Sewer Nemesis’s power and toughness are each equal to the number of cards in the chosen player’s graveyard.
Whenever the chosen player casts a spell, that player mills a card.
If the chosen player leaves the game, Sewer Nemesis's power and toughness each become 0. Unless another effect is raising its toughness above 0, it will be put into its owner's graveyard as a state-based action.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.