Memory Sluice

Conspire is a templating trick that lets a one-mana spell scale with a board you were going to build anyway, and pairing it with mill is the cleanest demonstration of why the mechanic exists. The hybrid blue-or-black cost is what makes the conspire clause workable: any two of your untapped creatures sharing blue or black with the spell can tap to copy it, so a self-mill or dedicated mill deck running either color always has bodies eligible to pay the additional cost. Cast on its own, four cards off the top is a rounding error. Tapped through conspire, eight cards (or four into one player and four into another, since the copy may choose a new target) starts to do real work toward filling a graveyard. It belongs to the line of cheap dedicated mill that runs from Glimpse the Unthinkable down through Mind Sculpt: a single spell that converts resources into cards in a yard, asking the deck to supply the surrounding engine. What separates this one is that its scaling is paid in tapped creatures rather than mana, which makes it a strange fit for a strategy that usually wants its creatures defending or its mana free. That friction kept it from ever finding a home: conspire asks for exactly the resource a mill deck is least eager to commit, since the bodies you tap to double the mill are the bodies you needed to survive while the graveyard plan came online.