Grinding Station

The untap clause is the entire engine here, and it is doing something more dangerous than the mill rate suggests. The activated ability taps and demands an artifact sacrifice to grind three cards off a library, but the second line refunds the tap every time an artifact enters the battlefield, which means each new artifact resets the engine for another activation. The card was built as a combo piece dressed up as a mill outlet: pointed at an opponent it shaves a library three at a time, but its real prize is pointed at yourself, emptying your own library or stocking a graveyard for whatever payoff is waiting on the other side of the loop. The whole thing turns on the word "enters" with no caveat about whose artifact or how it arrived: tokens, returns from the graveyard, and free recursion all untap it. So the kill is a closed circuit. Feed it an artifact that returns to play on its own, sacrifice that artifact to mill three, watch the untap trigger fire as the artifact comes back, repeat. The cost is never mana but the steady supply of cheap permanents to keep the chain feeding itself. That untap-on-trigger pattern is the same structural trick that powers later artifact loops, where each cycle pays for the next. With no recurring artifact to feed it, it sacrifices itself for a single three-card mill and goes home; hand it a permanent that loops back, and it becomes a combo kill that resolves the moment the pieces assemble.