Sewer Nemesis

Pointed at the right opponent, this enters as a body the size of their graveyard and then keeps feeding itself: every spell that player casts mills them a card, which is another point of power and toughness handed back to you. That is the central trick. The player you choose is also the engine that inflates the creature, so a control player sitting on a stocked bin or a spellslinger chaining instants and sorceries turns a modest four-mana / into a threat that climbs without further investment on your part. The chosen-player clause is the wrinkle worth dwelling on: this is a choice, not a target, so it ignores hexproof and shroud entirely, and once the creature resolves the choice is locked in permanently. The size and the recurring mill both shadow that one opponent, but only while the creature stays on the battlefield; remove it and the meter stops ticking, which makes keeping it alive part of the cost. There is a self-defeating tension built in, too: milling a graveyard-matters opponent fuels exactly what they want, while milling someone toward an empty library is its own slow clock. Among the black creatures whose size is borrowed from a graveyard rather than printed on the card, the recurring mill sets this one apart: less a snapshot of the bin at one moment, more a meter that climbs each time your chosen player keeps casting.