Slobad, Iron Goblin

Mana that only spends one way is the whole trick here: the red produced can go nowhere but artifact spells and artifact abilities, which converts what looks like a Goblin Engineer riff into a dedicated fuel line. The exchange rate is where the design gets exacting. The mana added equals the sacrificed artifact's mana value, no more, so it is a one-for-one conversion rather than a ramp step: cashing a Sol Ring returns exactly one red (its mana value is one, whatever it later produces), a Wayfarer's Bauble likewise returns one, and you have traded a permanent for a burst of color-locked mana. The engine only pulls ahead when the artifact you sacrifice was going to sit idle anyway, or when the mana it returns launches an artifact spell you could not otherwise reach this turn. The color lock does the balancing: you cannot cash a bloated rock into a game-ending spell in another color, only into more artifact velocity, so it stays a specialist rather than a generic sacrifice outlet like Ashnod's Altar. Because the ability taps, it fires once a turn and rewards feeding it one expensive artifact rather than a wide board of zero-cost tokens that would return nothing. Then there is the character reversal. The earlier Slobad, Goblin Tinkerer tapped to save an artifact from destruction; this Phyrexian version inverts him into a converter who feeds the machine rather than shielding it, a narrow, high-ceiling engine that a color-restricted mana ability is built to gate.