Umezawa's Jitte

The reason this Equipment broke the math on every creature it touched is the doubling. Most counter-based payoffs accrue one charge per trigger and force you to ration them; this one banks two per hit, so a single point of combat damage funds two activations, and any creature that connects twice is sitting on a stockpile that outlasts the game. That accumulation rewrites combat math from both ends at once: the -1/-1 mode clears blockers and assassinates utility creatures at instant speed, the +2/+2 mode pushes through whatever survives, and the lifegain mode buries aggressive opponents under a clock they cannot beat. A two-mana equip cost means it hops to a fresh body the moment its bearer dies, so removing the creature never removes the threat. The genius, and the problem, is that it is colorless and cheap enough to slot into any deck that attacks, and the charge counters make it a value engine rather than a one-shot pump: every connection compounds the advantage instead of spending it. Pro Tour formats spent years treating it as a near-mandatory inclusion in creature decks before it was banned in older Constructed formats, and its design has become the cautionary reference point for how a generically castable, recursive value engine on a stick warps board states. The restriction it lives within is thin: you have to connect, once, and after that the card does the rest.