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.
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.