Priest of Yawgmoth

The first card that turned artifacts into black mana, and the template Wizards has returned to ever since. The compression is what makes it dangerous: a Phyrexian cleric that converts any artifact's full mana value into a pool of black, which means the output scales with whatever you feed it. A sacrificed Memory Jar is five mana for one, and the math gets more lopsided as the artifact curve climbs; the higher the mana value, the better the conversion. (The trap is reading the cheap utility rocks as profit centers: Sol Ring has a mana value of one, so feeding it here returns exactly one black, no matter how much mana it makes while it lives.) The flavor is load-bearing too: an Antiquities-era Phyrexian sleeper agent whose ability is the literal Phyrexian thesis, that flesh and metal are interchangeable fuel. The tap requirement and the one-artifact-per-activation clause are the two design disciplines holding the rate in check; without summoning sickness and a real activation cost, the card would be a combo piece on rails rather than an engine that asks you to build around it. The lineage runs to Krark-Clan Ironworks, which traded the color restriction for colorless and the tap cost for a free activation, prefiguring exactly the kind of loops this card hinted at. This one set the ceiling and let the artifact's own cost do the scaling.