Sheltered Valley

One of the five legend-rule lands from Alliances, a cycle built on a self-destruct clause years before the legend rule applied to anything but legendary creatures. The replacement-effect wording is the trick: you can only ever have one in play, because the second sacrifices every other copy on its way in. That restriction is the entire balancing mechanism. The payoff (life gain capped to upkeeps where you control three or fewer lands) is calibrated to reward the small, mana-light decks of the era, and it switches off the moment your manabase grows past a deck that needs the help. It is a piece of design that taxes greed: the card pays you for staying lean, then quietly stops paying once you have built the kind of board that no longer cares about a single point of life. As a colorless mana source it asks nothing of your colors, which is what let the cycle slot into anything while keeping each individual valley a strict singleton. The whole package reads as a designer experiment in conditional, self-limiting value: a land that wants to be your only one of its name and rewards you, briefly, for keeping your ambitions modest.