Blanket of Night

This is a hate card pointed at a single seam in the rules engine: the line between a land's name and a land's subtype. By turning every land in play (yours and your opponents') into a Swamp, it lets black mana flow from sources that have nothing to do with black, but its real job is to weaponize cards that already punish Swamps. The era this came from was full of basic-land matters effects, and a symmetrical type-changer like this is the lever that makes a one-sided punisher land on everyone's board at once. The design logic is the same one that drives later type-fixers and color-screw enablers: the value is not in what the enchantment does on its own (nothing, really, until you build around it) but in the second card that cares about the type it grants. Note the precision of the wording: it adds the Swamp type rather than replacing what is there, so a Forest stays a Forest and still taps for green; nothing loses its existing function, it only gains a new one. That additive clause is what keeps the card from being purely a symmetrical Armageddon-by-color-screw and instead a quiet enabler that asks you to bring the payoff yourself.