Toxicrene

The 2/4 body with reach and deathtouch is a competent attrition wall, the sort of blocker fliers and ground attackers hesitate to trade into, but the wall is not the point. Hypertoxic Miasma is: it rewrites every land on every battlefield down to a single line of text, tapping for one mana of any color and nothing else, statically and with no target to counter or trigger to respond to. That is spell-less land-hate that flattens an entire ecosystem of utility at once. Manlands stop animating, sacrifice-for-value lands stop sacrificing, and the tap abilities that make treasure or draw cards or ping for damage all go dark while the ability is on the board. Even fetchlands stop fetching: a fetchland in play is still a land, so it loses its search ability under the clause and can only tap for any color, meaning what the Miasma denies is not the mana but the recurring incremental work a developed manabase was built to keep doing. The symmetry is where the design turns clever: your own lands become perfect rainbow fixing under the same clause, so the effect reads as a gift to a deck that only wants generic or five-color mana and a tax on opponents whose manabases were built to do more than produce it. It is stax that arrives dressed as fixing, and it hurts most the players who built their lands to do the most.