Sewer Nemesis is a rare Creature — Horror card from Commander Anthology Volume II (CM2), collector number 75. Current market price: $1.26, TCGplayer market prices via Scryfall.
As this creature 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.