Emissary of Grudges

• WWizards of the Coast2018-07-13
  Official Ruling
  If you didn’t choose an opponent when Emissary of Grudges entered the battlefield, you can’t pay the cost to activate its last ability. This will usually happen because you gained control of another player’s Emissary of Grudges or because a creature already on the battlefield became a copy of Emissary of Grudges.
• WWizards of the Coast2018-07-13
  Official Ruling
  If you have more than one Emissary of Grudges, you may choose a different opponent for each one. Be careful to keep track of which opponent is chosen for each Emissary of Grudges.
• WWizards of the Coast2018-07-13
  Official Ruling
  The activated ability of Emissary of Grudges can affect a spell or ability that has multiple targets, even if some of those targets aren’t you or permanents you control. You can choose new targets as long as at least one of those targets is you or a permanent you control.
• WWizards of the Coast2018-07-13
  Official Ruling
  There are several ways to secretly choose an opponent, including writing that player’s name on a piece of paper that’s kept with Emissary of Grudges.
• WWizards of the Coast2018-07-13
  Official Ruling
  If Emissary of Grudges leaves the battlefield and returns, it’s considered a new object. You may choose a new opponent and you may activate its last ability again.

Rulings via Scryfall