Densini's Radiance

You call down an explosion of radiant fire from the heavens on a point you can see within range. Each creature in a 15-foot-radius sphere centered on that point must make a Constitution saving throw. On a failed save, a creature takes 6d6 radiant damage and is marked with holy radiance until the end of your next turn. On a successful save, a creature takes half damage and isn't marked. 

The next attack roll against a marked creature before the end of your next turn has advantage.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.