Kagemaro, First to Suffer

Tying both halves of the card to the same number is what makes this design tick: the body is as big as your grip, and the wrath is exactly as deadly as that body. That coupling cuts in a precise direction. The sweep gets stronger the fuller your hand, but the toughness rises in lockstep, so the cleaner the kill, the larger the creature you just sacrificed to deliver it. Hold a single card and the -X/-X shrinks to a -1/-1 twitch; hold seven and you wipe most boards while throwing away a 7/7 to do it. The sacrifice clause is the honest part of the deal: the price that stops a recurring hand-sized body from also being a recurring hand-sized sweeper. Crucially, the cards stay in your hand. The ability only asks for one black mana and the body itself, so the grip that powered the wipe is still there to rebuild the turn after. That also resolves the timing problem black has always had with mass removal: a creature that doubles as a one-shot board wipe at instant speed, holding the activated ability back until an attack commits or a combo assembles. Demons in this color have a long line of sweepers paid for in life or in discarded cards; this one settles its bill in its own corpse, leaving the hand that gave it size untouched.