Karn

Every noncreature artifact you control wakes up as a creature scaled to its mana value, which makes this avatar read less like a buff and more like a global rules patch: a Mox becomes a 0/0 and dies on the spot, a Sol Ring becomes a body, and the expensive stuff you cast for value suddenly swings. The scaling cuts both ways, and that is the whole design. Zero-cost mana rocks become 0/0s that fall over instantly, so the avatar punishes the cheap acceleration most artifact decks are built around and rewards a suite of mid-and-high-value artifacts instead. It comes from the Vanguard experiment, a short-lived format where players chose an avatar that altered the game's baseline rules rather than sitting on the battlefield as a permanent, so there is no cost and no body here, just a static line that says your artifacts are alive now and as big as they were expensive to cast. It is an early, blunt sketch of an idea Wizards would keep refining: the Karn name later landed on a Planeswalker and a creature whose artifact-animation effects targeted with far more control. This is the crude prototype of all of that, an animation engine that asks nothing and gives no nuance, just a sweeping rules rewrite that turns mana value into power and toughness across the board.