this.addAbility(new MonstrosityAbility("{5}{R}{R}", 1));
// When Wildfire Cerberus becomes monstrous, it deals 2 damage to each opponent and each creature your opponents control.
Effect effect = new DamagePlayersEffect(2, TargetController.OPPONENT);
effect.setText("it deals 2 damage to each opponent");
Ability ability = new BecomesMonstrousSourceTriggeredAbility(effect);
effect = new DamageAllEffect(2, filter);
effect.setText("and each creature your opponents control");
ability.addEffect(effect);
this.addAbility(ability);
}