Slaughter

The buyback mechanic asks a single question of every card it touches: what is the recurring cost worth paying to keep a spell in your hand? Here the answer is paid in life, four points at a time, and that pricing is the whole design. Most buyback spells of the era recurred for mana, which let you spend a turn doing one repeatable thing. Paying life instead frees the mana cost to stay flat while letting the controller bleed themselves down to keep a removal spell on a loop, turning a four-mana instant into an attrition engine that answers nonblack creature after nonblack creature without ever leaving the hand. The constraint that makes the rate work is the color exemption: black creatures walk free, which keeps Slaughter from being a universal answer and ties its reach to the texture of the board. The "can't be regenerated" clause closes the obvious escape hatch, so the destruction is final against the things it can hit. What you are really buying with that life total is permanence of access, not permanence of effect; the spell evaporates the moment you decline the buyback, so the engine runs exactly as long as your life can sustain it. That self-imposed clock is the elegant part: a removal loop that prices its own ceiling into the life total, then dares you to find the bottom before the board does.