Floodgate

A defensive wall whose payoff is the opposite of defense: parked on the battlefield it only blocks, but the instant it leaves it detonates, scaling damage off your Island count to every nonblue, nonflying creature on the board. The design hangs on the leaves-the-battlefield trigger rather than the body, so a 0/5 Wall becomes a delayed sweeper instead of a brick. The two self-referential clauses are the cleverness. The first sacrifices the Wall the moment it gains flying, a strange-looking restriction that exists to keep you from animating or evading it past the second clause's "without flying" filter, and to give a controlled way to fire the trigger on your own terms. The second is the bomb: a one-sided board sweep that spares your own blue creatures and anything airborne, paid for entirely in lands rather than mana. The fuel is any permanent with the Island land type, so dual lands and other nonbasics with that subtype count the same as basics; the more of them you run, the harder the blast lands. The construction asks something of both halves of your deck. A heavily Island-based mana base makes the math hurt, while keeping your own threats blue or flying turns the blast into asymmetric removal rather than a symmetric Pyroclasm. The friction is that you must actively remove your own Wall to collect, so a passive defender becomes a sacrifice-fueled wrath waiting for a bounce spell, a sacrifice effect, or simple combat death to set it off.