Cleansing flames

upon saying the incantation, a blast of holy flames bursts forth, dealing 3d8 radiant damage to all creatures hit, or half that if they fail a dc 14 dexterity saving throw. You may choose to change the damage type to fire instead of radiant when casting the spell. Undead creatures and demons take double damage from this spell. This spell does an extra 1d8 damage for every level above the first.