Doubling Cube

Mana doubling is a deceptively dangerous design lever, because the payoff scales with the floor: pay three to activate, double whatever is left, and the surplus compounds the more you already have. That is the whole calculus. With an empty pool the three-mana activation is a dead nothing; with eight mana floating it turns a big-mana turn into a ridiculous one. The tap ability makes it reusable, but the timing rules render that almost irrelevant: unspent mana empties at the end of each step and phase, so the doubling only ever matters inside the same window where you intend to spend the result. That is what makes it a within-turn burst converter rather than incremental ramp. It does not grow a board across turns or bank mana for later; it amplifies the single explosive turn you have already built toward, then waits to do it again the next time you flood. To use it you have to generate mana faster than you can spend it, find the floating reserve, and tap into the surge to cross a finish line. The design tension is honest about itself: it rewards excess and punishes restraint, a multiplier that only multiplies what you have already worked to accumulate. It treats mana as a snowballing resource rather than a per-turn allotment, and prices that fantasy at exactly the moment you can least afford to waste a card slot on it.