Allatos' Purifying Flame

As part of the casting action the caster makes a Melee Spell Attack Roll to touch an enemy. If successful, the touch sets a purifying flame alight within the target. They immediately takes 3d6 points of fire damage. While the spell remains active, at the start of the caster's turn the target must succeed on a Constitution Saving throw or take an additional 3d6 fire damage.  This continues until they succeed on the save, the max duration is reached or the cleric ceases to concentrate on the spell.