Riverchurn Monument

Two mill buttons in one shell, sequenced so the second only pays off after the first has done its work. The tap-for-two half is the slow-drip engine: a repeatable clock that fills graveyards a couple of cards at a time, most of them your opponents' but yours too if you want them. That accumulation is the setup for the exhaust half, which mills each target equal to the cards already sitting in their graveyard. Fire it early against a fresh library and it does almost nothing; fire it after the board has been grinding through cards for a while and it can double a graveyard in a single activation, a payoff scaled entirely by how deep the game has run. The one-time exhaust clause is what keeps that scaling honest: you get a single detonation, so the question is never whether to use it but when the accumulated graveyards are large enough to make it lethal or close. The multi-target wording (any number of players each mill) turns it into a symmetric threat at a full table, where every opponent's graveyard becomes ammunition against them. It wants blue mana for the exhaust half, the second activation demanding a double-blue commitment, so this is a dedicated engine for a blue mill deck willing to spend a card on a self-referential clock, not a colorless splash. The longer the board sits, the more the exhaust ability converts other people's discard piles into their own undoing: a mill design that pays out patience in a single burst.