Darkest Hour

The whole card is a single hinge: it makes every creature black, and the only reason that sentence justifies an enchantment slot is what black has historically been allowed to do to itself. Plenty of effects punish or reward creatures of a chosen color, and most of them lean on the assumption that opposing boards will not all share that color. Darkest Hour removes the choice. Pair it with anything that drains, sacrifices, or empowers black creatures, and effects meant to be conditional become unconditional. The card is a setup piece, not a payoff: on its own it changes nothing about the game state, which is the trade Wizards made to keep a one-mana global color-change from being oppressive. Its real legacy is as a combo enabler, the enchantment you name when you want a symmetrical board to suddenly answer to black-matters triggers. That role is the whole reason this otherwise inert one-liner has stayed in circulation: it is a key, and the lock is somebody else's card.