Detonating Smite

Upon hitting an enemy with a melee attack cast this spell and create a fiery emanation with a 20ft radius. The creatures hit by the emanation take an extra 3d6 Fire damage from the attack, and have the Burning condition until the spell ends. At the start of the Burning creatures turn the target takes 1d6 Fire damage. At the end of each of its turns, the Burning target makes a Constitution saving throw, ending the spell on itself on a success. Alternatively, the Burning creature can be put out using magical means.

Using a Higher-Level Spell Slot. The extra damage increases by 1d6 for each spell slot level above 3. The burn damage does not increase at higher spell levels.