Tsunami

Four mana, no targeting clause, no exceptions: green erases every Island on the battlefield, and the original design team considered that an honest trade. This is one piece of a mono-color land-destruction cycle (Flashfires did the same to Plains) that treated cross-color hatred as a baseline costing assumption rather than a problem to solve. The logic is brutally direct. You leaned on Islands; you lose them all, legendary or nonbasic or otherwise, with nothing to do about it. Modern design has retreated almost entirely from this space. The closest descendants either moved to red (Boil, Boiling Seas), gained instant speed at a premium, or swapped destruction for tapping (Choke), because Wizards eventually decided that punishing a player for their land choice with zero interaction was bad for the game. The card sits at the far end of that retreat: an artifact of the era when a sideboard slot could simply read "you lose if your manabase is Islands," and the design team considered four mana and a blank card against most opponents a fair price for the asymmetry.