Karn, Legacy Reforged

A five-mana body that reads its own stats off the room: power and toughness each set to the greatest mana value among your artifacts, so the golem grows as your board's biggest machine does. The upkeep trigger keys off a different number entirely, adding one colorless for each artifact you control, and that split is the clever part: the body wants one enormous artifact, while the ramp wants a swarm of any size, and a well-built shell can feed both axes at once. The mana the trigger makes comes with a leash, though: it cannot pay to cast nonartifact spells, and it holds through the end of the turn instead of draining as phases pass. That restriction narrows where the flood can go while leaving real range: it still pours into artifact spells, but also into activated abilities, ward costs, and other non-spell payments, so the engine can crack, equip, and activate on its own colorless without ever touching your other lands.

The design lineage runs through the various Karn incarnations that reward an artifact-dense board, but the mechanical trick here sits closer to Mishra's Workshop logic than to any planeswalker: a large, type-locked mana source whose ceiling scales with how much metal you have already committed. The leash is the price; a body that measures your biggest artifact and a battery that counts all of them is what you buy with it.