Atomic Atonement

The breath weapon of the mighty king of monsters takes form in this lesser being.  You fire this inferno from your mouth and hit every creature within a 5 ft x 50 ft line. Creatures in this range must make a Dexterity Saving Throw against your spell save DC, on a failed save, creatures take 10d10 radiant damage, half as much on a successful save. The targeted point within range will explode in a 5 ft radius dome of radiation and death. Creatures within this explosion range must also make the saving throw to avoid damage. 

At Higher Levels: When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d10 for each slot level above 5th.