Blue Flame Burst

You speak the power word and blue flames conjure into both of your palms. Make a ranged spell attack. on a hit the target takes 5d10 fire damage. when you cast this spell you use all your movement for that turn but you gain temporary fire immunity till the end of your next turn. On a successful hit a blue light will mark the creatures heart or head. if it has neither it will illuminate bright blue all over. The creature must maker a CON saving throw or burst into blue fiery flames and take 5d10+ 15 fire damage as flames burst from their insides. A creature dropped to 0 hit points by this spell will combust and cause a fiery explosion in a 15 ft. radius, any creatures  caught within this blast must make a DC 14 DEX save or take 4d6 fire damage. If you break concentration in between the time you initially made the spell attack and your next turn the combustion will not take effect.

At Higher Levels. For each level above 6th the damage for the combustion increases by 1d10+5 and the explosion damage increases by 1d6+5.