Keening Stone

Most mill engines shave a fixed slice off the top of a library and ask you to repeat the activation enough times to matter. This one runs on a feedback loop instead: each tap mills a number equal to what already sits in the target's graveyard, so the very cards you put there feed the next activation. The first fire into a clean library does nothing; once another effect has primed the yard, the count snowballs, five into ten, ten into twenty, with the curve bending toward emptying the deck outright. That dependency is the entire design. The card is deadweight early and lethal late, a closer rather than a clock, useful only once you have dug the target's graveyard deep enough to reach for it. The hefty activation cost is the throttle that enforces this, expensive enough that you never want to fire it while the yard is shallow, which steers the card squarely into its intended role as an endgame finisher. The payoff is a single tap that buries the rest of a deck and turns the kill into bookkeeping. Among self-accelerating answers in the mill space it stands nearly alone, colorless enough to slot into any deck, and it scales not with how many copies you draw but with how much of the target's graveyard you have managed to stock before you activate it.