Holy Fire

Prerequisites: Dragonborn race, Paladin Level 8

Your righteousness as a paladin seeps into your draconic blood and lets you infuse your breath with divine energy.

You can choose to expend a spell slot to add 2s8 radiant damage for each spell level of up to 3d8 radiant damage to your breath weapon.   
Holy Fire deals an extra 1d8 radiant damage up to 4d8 if the target creature(s) is a fiend or undead.

This effect applies to the breath weapon as a whole and thus does not need to be used for each creature damaged by the breath weapon.
Holy Fire acts as the Divine Smite special action and thus can be spent after the damage calculation.

"Let Evil burn away and become a torch of hope for those in dismay. Be the light in eternal darkness, my Champion!"