Kazarov, Sengir Pureblood

The trigger is the design's whole engine, and it is broader than a casual read suggests: any damage to any creature an opponent controls grows this body, not just damage Kazarov itself deals. Combat damage from your other attackers, a board wipe that burns, pingers, fight spells, anything that touches an enemy creature stacks a counter. The built-in ability supplies a renewable, repeatable source of that damage, two at a time, and conveniently it can fire at the same creatures over multiple turns, so the activation doubles as a slow grinding wrath and a growth pump in one line. That red activation cost is the structural tell: this is a Vampire that explicitly demands a second color to run its own motor at full tilt, a Rakdos build-around dressed as a mono-black finisher. Left to its own devices in a pure black shell, the trigger sits idle until something else deals the damage; paired with a removal suite or an aggressive board, the 4/4 flier escalates into a real clock fast. The flavor lines up with the math, too: the patriarch of the Sengir bloodline feeding on the wounded, every drop of spilled blood making him larger. It is a payoff card that asks you to build a damage-dealing board around it rather than a self-contained threat, and the wider you read the trigger, the more sources of growth you find already sitting in your deck.