Massacre Wurm enters the battlefield, giving all opposing creatures -2/-2 until end of turn.
Because Living Plane causes all lands to be 1/1 creatures, all of your opponent' lands die as a state-based action due to having zero or less toughness.
Massacre Wurm's second ability triggers for each land that dies this way, causing each opponent to lose 2 life for each land that died under their control.