Massacre

The free-cast clause is the entire design, a sweeper that lives or dies on what the table looks like across from you. The printed price asks four mana for a symmetric board reduction, a rate that reads as unremarkable. But when an opponent controls a Plains and you control a Swamp, the cost drops to zero: you strip two toughness off every creature and pay only the card. That conditional turns a mediocre sorcery into a resource blowout that scales with how committed your opponent is to small bodies. The asymmetry was deliberate anti-white technology, priced for an era when weenie aggression leaned on Plains, and the permission rewards you precisely when the opponent has done the work of presenting targets. The discount is not reduced mana but a position both manabases must conspire to create: the opponent's Plains and your own Swamp must both be present, so the bargain is a board state you have engineered rather than a cost reduction you have unlocked. The catch is twofold. The -2/-2 hits your own creatures too, so the free cast only pays off when you hold an asymmetric battlefield or are willing to trade your side away; and because it resolves at sorcery speed, you cannot sandbag it for a combat blowout or hold it as a response to a developing board. You commit on your own main phase, into whatever the opponent has chosen to leave standing.