Lodestone Golem

Taxation as a body, not a permanent that sits back. The Sphere of Resistance effect (every nonartifact spell costs one more) usually lives on a cheap, fragile artifact you protect; this welds it to a 5/3 that ends games on its own clock. That dual identity is the whole strategic axis: the tax slows the opponent's interaction and combo enablers while the body kills them before the surcharge stops mattering, which is exactly the asymmetry artifact-heavy aggro decks want. The wrinkle is that the tax is symmetric, and the 5/3 line is the deckbuilder's escape hatch from its own lock: because the body presses the clock, you can afford to spend mana taxing both players rather than building a soft prison and stalling out. The fragility is the cost. A 5/3 is a real threat but a soft one; it dies to most removal and trades down in combat, so the tax only holds for as long as the golem survives, which turns every game into a race between the surcharge and the opponent's answer for it. Most cards that impose a Sphere-style tax are noncreature permanents precisely so they cannot be killed in combat or with a creature-removal spell; stapling the effect to a creature trades that durability for pressure, and decides who that trade is good for.