O-Naginata

The power-3 attachment clause reads like a restriction until you notice it is actually a discount. Equipment that hands out a flat +3/+0 with trample for a one-mana investment and a cheap equip cost would be flatly overpowered if it could go on anything; the gate narrows it to creatures already built to swing, so the card stops being a way to make a small threat relevant and becomes a way to make an existing beater lethal. That is the deliberate trade: the price you pay for the aggressive rate is that it sits dead in the early turns when you have only one- and two-power bodies, and turns on the moment your fatties land. Trample is what tips the +3 from incremental to game-ending, since a chump blocker can no longer absorb the swing. The samurai-naginata flavor matches the mechanics cleanly: this is a polearm for someone who is already a warrior, not a weapon that creates one. It rewards a board built top-down around a few large bodies rather than a wide one, a narrower deckbuilding ask than most Equipment makes, and the payoff for meeting it is among the steepest power boosts ever stapled to a sub-three-mana artifact.