Krark-Clan Ironworks

The conversion is the whole point: any artifact, regardless of what it cost to cast, becomes two colorless mana on the way to the graveyard. That single line turns artifacts from objects you spend mana on into objects that store mana, and the gap between those two ideas is where the combo decks live. Feed it a zero-cost artifact and a way to return that artifact to play for less than two mana, and you have built a loop that nets colorless every cycle: the engine is not the card itself but the math that opens up the moment a free artifact and a cheap recursion outlet sit on the same battlefield. That is exactly the math that earned it a Modern ban, the format's banlist citing a deck that used this to power out lethal storm-style turns. What makes the design durable is that it asks for so little: no tap, no per-turn limit, just fuel. Hand it enough disposable permanents and it produces mana faster than most decks can produce threats, which is precisely the property that makes it dangerous and precisely the property that makes it self-limiting (an empty board is a dead rock). It is the rare ramp piece that scales not with how much mana you have but with how many artifacts you are willing to throw away, and the decks that abuse it are always the ones with the most disposable junk.