Horizon Stone

Empty mana pools between steps are one of the game's oldest housekeeping rules, and the reason big-mana engines have always leaked value: floating mana that goes unspent simply vanishes at the end of a phase. This converts that penalty into a savings account. Colorless is the tax it charges: whatever pours out of Cabal Coffers, Nyxbloom Ancient, or a doubled-up Gaea's Cradle keeps existing, but it becomes colorless mana you can only feed into generic costs and dedicated costs. That restriction stops the effect from being a strictly-better mana rock: you cannot bank blue for a counterspell or black for a reanimation, only stockpile fuel for X-spells, activated abilities, and colorless-hungry payoffs. The strategic axis it opens is the two-turn burst: dump every ritual and land-untapper you have this turn, then cash the reserve on your next, or across the same turn's phases when a huge mana pool would otherwise time out before you can spend it. It turns a one-time explosion of mana into a persistent resource, and the only price of admission is a deck already built to overproduce.