Foggy Bottom Swamp

A dual land that folds a mana sink into its own destruction: it enters tapped, produces black or green for as long as you keep it, then cashes itself in for a card once your mana is deep enough to spare four and the land itself. The lineage runs through the karoo lands and every other design that asks a fixing land to earn its slot twice, but this one takes the bluntest path: the land is the fuel. Entering tapped is the price paid for the untaxed color it gives you afterward, the standard rate of this era's dual-color manabase design. What sets the sacrifice mode apart is its timing. Late in a game where flooding turns extra lands into blanks, the fourth or fifth source of your colors stops being mana and becomes a draw step you already paid for. The land does not replace a spell in your opening hand; it converts surplus land into gas on the turns you would otherwise be discarding it. It fixes freely while the game is young and becomes a mana-hungry cantrip only once you have the mana to burn, at which point it removes itself from the board rather than lingering as a dead permanent. It asks nothing of the deck around it beyond wanting black and green, which is exactly the point of a card built to smooth the curve at both ends.