Drought

Black does not pay its costs in mana alone here: every black pip in a spell or activated ability demands that a Swamp be sacrificed, so a Drain Life or a Dark Ritual stops being a question of having the mana and becomes a question of how much of your manabase you are willing to dismantle. Double-black cards eat two Swamps; the deeper a deck leans into black, the more it pays. That is the elegant part of the construction: the tax scales with an opponent's commitment to the color rather than imposing a flat fee. The upkeep clause is the self-limiting valve. Holding the lock open costs two white every turn, which means the controller taxes their own development to keep the enchantment online, and against a non-black deck the thing simply dissolves. Ice Age was full of these asymmetric color-hosers, pacts that could brick entirely against the wrong opponent: a sideboard answer dressed as a maindeck enchantment, brutal against the decks it was aimed at and inert against everything else. The mechanic of paying costs in sacrificed lands rather than mana is the through-line worth noting; it converts a color's identity into a resource its own spells consume, turning the Swamp count that powers black's spells into the exact thing the enchantment bleeds dry.