When this creature enters, creatures your opponents control get -2/-2 until end of turn.
Whenever a creature an opponent controls dies, that player loses 2 life.
Massacre Wurm's triggered ability triggers if a creature an opponent controls dies for any reason, including from its first ability reducing its toughness to 0, as long as Massacre Wurm remains on the battlefield.
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wotc
Massacre Wurm's first ability affects only creatures your opponents control at the time it resolves. Creatures they begin to control later in the turn won't get -2/-2.
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wotc
If your life total is brought to 0 or less at the same time that a creature an opponent controls is dealt lethal damage, you lose the game before Massacre Wurm's last ability goes on the stack.
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wotc
If a creature an opponent controls dies at the same time as Massacre Wurm, Massacre Wurm's last ability triggers for that creature.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.