Circu, Dimir Lobotomist

Circu, Dimir Lobotomist sees essentially all its competitive and casual play in Commander, where the multiplayer mill-and-exile effect hits every opponent simultaneously and infinite spell loops can close games outright. In Legacy and Vintage, the effect is too slow and too conditional — four mana for a creature that mills one card per spell cast is never the plan when those formats close in two or three turns. Modern technically allows it but the same problem applies: the effect doesn't trade resources efficiently enough against the format's speed. Commander is the correct home, specifically in high-density blue-black spellslinger builds or combo shells where Circu's triggers are a natural output of the game plan rather than a cost.