If another effect (or effects) modifies how much damage a source you control of the chosen type would deal (for example, by preventing some of that damage), the player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects, including the effect of Collective Inferno's last ability, apply. If all of the damage is prevented before the effect of Collective Inferno's last ability would apply, its effect no longer applies.