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. In a Two-Headed Giant game, each creature controlled by either member of the defending team will get -1/-1.