Silumgar, the Drifting Death

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

• WWizards of the Coast2014-11-24
  Official Ruling
  The creatures affected by Silumgar’s triggered ability are determined relative to the Dragon that attacked. For example, if Silumgar attacked one opponent and two other Dragons attacked another opponent, creatures controlled by the first opponent would get -1/-1 until end of turn and creatures controlled by the second opponent would get -2/-2 until end of turn.

Rulings via Scryfall