abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
in its mana cost, X is considered to be 0.Karn, the Great Creator can shut down opposing artifact abilities while leaving your own artifacts functional. Mana rocks, Treasure tokens, Equipment activations, and many artifact engines become harder to use while Karn remains in play. Its static ability is often the main attraction even before you activate a loyalty ability.
Mycosynth Lattice expands that restriction by making every permanent an artifact. Together, the cards prevent opponents from activating abilities of their lands and other permanents, including ordinary mana abilities. The combination is powerful but requires a six-mana artifact alongside Karn, and opponents can still use resources and actions that the restriction does not cover.
Karn's plus ability animates a noncreature artifact according to its mana value. That can create an attacker or turn a mana-value-zero artifact into a creature with zero toughness, subject to other effects. Be aware that animating your own artifact can expose it to creature removal and summoning-sickness restrictions.
The minus ability can recover an artifact you own from face-up exile. Its outside-the-game option depends on the game's agreed rules; ordinary Commander games do not automatically provide a sideboard to search. Even without that route, recovering an exiled artifact can be valuable. Build around the abilities available in your game rather than assuming the card always comes with a toolbox outside the deck. Artifact-heavy strategies appreciate the asymmetry, but Karn still needs protection from combat.
