Flesh Reaver

Black almost never gets to deploy a body this large for this little, and the oracle text explains the discount: every point of damage it deals to a creature or opponent comes straight back to the controller. The result is a clock pointed in both directions. Trade it into a blocker and you absorb whatever it dealt; connect with the opponent and you take that much again. The body that should be closing games is also bleeding the player who cast it. The detail that keeps the drawback from ever softening is that the recoil mirrors damage dealt, not the creature's base power, so pumping it makes the math worse: a bigger swing buys a bigger backlash. The way out is not hitting harder but breaking the return loop. Layer a damage-prevention effect on yourself and the downside vanishes outright; treat the creature as expendable, throwing it at a blocker to trade and eating the recoil on purpose, and that becomes a cleaner line than swinging into the open. This is the strong-but-self-harming Phyrexian idiom in miniature, where the threat to the controller is the cost rather than the mana. It treats your life total as a payment schedule, which only works for a deck that already counts life among its resources rather than its defenses.