Planar Collapse 
Enchantment
At the beginning of your upkeep, if there are four or more creatures on the battlefield, sacrifice this enchantment and destroy all creatures. They can't be regenerated.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL