Mirrorworks

The pivot point where artifact decks tip from incremental value into runaway engine. Most copy effects from this era were one-shot spells or attached themselves to a single permanent; this one sits in the air as a standing tax, asking nothing but two mana per artifact you would already be casting. The taxed-trigger structure is what keeps it from breaking outright: every copy costs real mana, so a turn that floods the board with artifacts drains your pool faster than you can pay, and choosing which entry to double up on becomes the actual game. The token clause does quiet but decisive work, because the copies it makes are tokens and so never re-trigger Mirrorworks itself: no infinite cascade off a single artifact landing. What it loves is artifacts whose value lives in the enters-the-battlefield window rather than the body, since you are paying to relive that moment rather than to maintain a creature. It sits alongside the rare effects that turn each artifact you play into two without combat, two cards, or two casts, and the design has aged into a reliable backbone for any build that treats artifacts as the resource rather than the support: a five-mana commitment that only pays off after you have already committed to the plan.