Bosh, Iron Golem

The plane's golem mascot doubles as its central artifact cannon: feed Bosh an artifact and he flings damage equal to its mana value at anything you point him at. The 6/7 trample body comes with such a steep entry fee because the cannon scales with the junk you're sacrificing. A spent Darksteel Forge, a leftover mana rock, a creature that has done its job: each one becomes a bolt, and on a board littered with high-cost artifacts the numbers get absurd in a hurry. The repeatable cost separates Bosh from a one-shot artifact-flinger like Shrapnel Blast: you can keep firing every turn you have artifacts to burn, which turns a long game into a slow-drip removal engine that also closes through chump blockers. The tension is that Bosh wants two things at once, an artifact-dense board to fuel the gun and the colored mana to keep pulling the trigger, and the eight-mana investment to even get him down means the engine usually comes online late. That late timing is the cost of an effect that can reach any target, creature or planeswalker or face, without caring about combat. Bosh is the kind of payoff a set built around artifacts has to print eventually: the legend who turns your accumulated metal into a finishing sequence.