Nightcreep

Two reshapes ride on one cheap instant, and each one is its own kind of ambush. Setting every land to Swamp does not unlock off-color spells: it overwrites their other land types and mana abilities, so for the rest of the turn the entire battlefield produces black and only black. That is a clamp on an opponent's mana base as much as a feeder for your own Swamp-counting payoffs. The timing is subtler than a blunt "tap them out," though, because mana abilities cannot be responded to: cast in their upkeep or draw step, before any colored mana can be floated into the main phase, and the spells that needed colored mana you just erased simply cannot be paid for. Turning every creature black is the colder half, and it cuts both ways. It hides your own team from anything that spares black while exposing their creatures to effects that specifically punish black, the kind that read "destroy all black creatures." Instant speed arms both lines: the color flip can land in response to targeted removal aimed at a creature the opponent thought was safely off-color, and the global rewrite slots into the gaps in their plan. As color-changing tech this descends from the enchantment school of Conversion, but where those bet on a standing, permanent advantage, this trades durability for surprise. The change lasts only through the current turn, so it is built to steal one exchange rather than to grind. That narrowness is the price of a board-wide rewrite for so little: it wants a payoff already in hand.