Sealock Monster

The attack restriction is the whole joke, and the monstrosity trigger is the punchline. Here is a 5/5 for five that can't swing unless the defending player controls an Island, a clause that would render it inert against most opponents in most games. Then the activated ability resolves and the creature solves its own problem: becoming monstrous turns a target land into an Island, manufacturing the exact condition the attack restriction demands. You spend the seven mana to grow it to an 8/8, and in the same breath you Islandify a land the defender controls so the giant octopus can finally come ashore.

It is a self-contained two-card combo folded into a single shell, and because the type change has no duration, the timing is more flexible than it first appears. The land stays an Island permanently, so the patient line is to fire monstrosity on a quiet turn (or on an opponent's end step) and clear the path well before the creature ever needs to attack. Designs that gate combat behind a board condition the controller doesn't normally hold (an Island, a particular permanent) usually leave that condition to chance or to another card; this one bakes the key and the lock into the same body. The flavor lands cleanly too: a sea monster that can only assault land it has already flooded, dragging the coastline toward it before it strikes.