Blazing Crescendo

You target a creature within range forcing them to make a Constitution saving throw, on a failure a temporarily dormant flame is created within them, on a success they take 2d6 fire damage. When you cast the spell you choose a duration of 1 to 10 rounds, once the spell has been cast it cannot be cancelled and the duration cannot be modified. After the set amount of time passes, flames explode from the creature out to 10 ft. dealing 10d6 fire damage to the creature. Every other creature within range must make a Dexterity saving throw or take 6d6 fire damage, half on success.