Murderous Betrayal

The price is the design. Repeatable creature removal that ignores both color (almost) and regeneration would be oppressive at a flat mana cost, so the activation is priced in a currency that escalates against itself: every use halves your remaining life, rounded up, which means the engine throttles harder the more you lean on it. From twenty life, the first kill costs ten; the second costs five; the curve bottoms out fast. That self-correcting payment is what lets the effect be repeatable at all, and it ties the card to a Phyrexian-flavored masochism where life was treated as a resource to be spent rather than a number to protect. The nonblack clause is the other guardrail, exempting black's own threats while leaving every other creature exposed, so the engine never turns on the deck most likely to host it. What it asks of the pilot is brutal arithmetic: the removal is always available, but the life total is the meter, and a player who clears too many creatures has paid for those answers with the very buffer that kept them alive. It is removal that wins the board and loses the race, a tension the card never resolves on your behalf.