Lazav, Dimir Mastermind

• WOTC Official Reminder Textofficial rules
  Hexproof — This permanent can't be the target of spells or abilities your opponents control.

• WWizards of the Coast2013-01-24
  Official Ruling
  If multiple creature cards are put into an opponent's graveyard at the same time, you choose the order that Lazav's triggered abilities go on the stack.
• WWizards of the Coast2013-01-24
  Official Ruling
  Token creatures dying won't cause Lazav's triggered ability to trigger.
• WWizards of the Coast2013-01-24
  Official Ruling
  Lazav, Dimir Mastermind becomes a copy of the creature card in the graveyard. If that card is no longer in the graveyard when Lazav's ability resolves, use the characteristics of that card as it last existed in the graveyard. Notably, copy effects that applied to the creature card when it was on the battlefield won't be copied by Lazav. For example, if a Clone is put into an opponent's graveyard from the battlefield and you use Lazav's ability, it will become 0/0. It won't become a copy of whatever Clone was copying.
• WWizards of the Coast2013-01-24
  Official Ruling
  "Enters" abilities of the creature card Lazav is copying won't trigger as Lazav is already on the battlefield when it becomes a copy of that creature card.

Rulings via Scryfall