All creatures get -2/-2 until end of turn. Whenever a creature dies this turn, you gain 1 life.
9/20/2014 You gain 1 life for each creature that Death Frenzy causes to have a toughness of 0 or less. The delayed triggered ability is created before state-based actions cause any creatures with 0 or less toughness to die.
9/20/2014 Only creatures on the battlefield when Death Frenzy resolves will get -2/-2. However, if a creature enters the battlefield and then dies later that turn, you’ll gain 1 life for it.