Forgotten Monument

The Cave subtype was mostly land-slot texture until this printing gave it a reason to congregate. On its own the tap ability produces a single colorless: unexciting, the price of admission. The payoff is what it does to every other Cave you control, retrofitting them into pain-lands that fix any color for a life. That is the design pivot: rather than fixing your mana itself, it turns a shared subtype into a fixing engine, so the value scales with how many Caves are already on the battlefield and rewards a manabase built to lean into the type. It sits in the same structural tradition as anthem lands, the ones that grant an ability to a class of permanents rather than acting alone (think of how Gaea's Cradle cares about a board rather than a single tap), but pointed at fixing instead of ramp. The life payment is the throttle that keeps the ability from being free five-color fixing: every activation of an adjacent Cave costs a life, so the engine taxes you the more you rely on it. What makes it more than a colorless-only land is that it converts an otherwise flavor-only land subtype into a deckbuilding axis, asking you to hoard Caves not for what each does but for what this one does to the group.