Massacre 
Sorcery
If an opponent controls a Plains and you control a Swamp, you may cast this spell without paying its mana cost. All creatures get -2/-2 until end of turn.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL