Invoke Prejudice

Quadruple-blue is the load-bearing design choice, and everything else follows from it. The cost forces a mono-blue (or near-mono-blue) deck, which means the controller's board tends toward a single color, which means the tax fires on most opposing creatures by default: anything that doesn't share a color with a creature the controller already has out gets countered unless the caster pays its own mana value a second time. The result is a permanent, asymmetric tax that doubles the cost of off-color threats. Because the enchantment is a standing permanent that sits on the battlefield, there is no playing around it: every applicable creature spell triggers it, so the opponent cannot resolve a cheap decoy to clear the way before slamming the real threat. Each one pays the toll in turn.

The card's reputation outside the rules is the harder thing to write around, and worth naming plainly. The title, the art (a hooded tribunal sitting in judgment over a shackled figure), and the mechanic (punishing creatures that fail to match your color) combined into one of the most notorious cards in the game's history, and it has sat on Wizards' informal do-not-reprint list for decades. Read coldly, the design is a clean piece of mono-blue prison tech, kin to Kismet and Meekstone and the stax pieces that came after; the presentation is why no one ever talks about it that way.