It is a composite skill of Area Boundary, Fire Manipulation, and Black Lightning. First, the space is secured with Area Boundary, He accelerates the molecular motion inside with Fire Manipulation. So it will produce a high temperature. Finally, the magical power inside the space acts as fuel, and the Black Lightning turns it into plasma that burns whatever is inside.
It can be said that the skills had become the composite skill Black Flame Manipulation. That skill, unlike a nuclear explosion, its characteristic is not causing any damage outside of its radius. Even if the boundary is released, there is no shock wave that goes outside. Range specification can be performed, it seems the purpose for increasing the inside heat synergistically is to make the heat inside unbearable. So once confined inside the boundary, there is no hope for survival.
To perform the attack Benimaru produces a black spherical flame in his right palm. Increasing the diameter of this ball to a meter, he shoots it at the target. The ever-accelerating round is able to chase any fleeing enemies. It's speed is around 600 kph. Those struck are immediately engulfed and burned in a 2,000 ~ 5,000 degree fireball, without leaving ashes. However, the terror of the flame was not in this. The ball also absorbs the magical energy of the enemies struck and uses it to fuel its growth. Rather than a ball, it turns into a dome with a radius of 100 meters.
As I expected it can't be fire rapidly, but it is doesn't require a large amount of magic power.
With the support of Great Sage, it's possible to reduce the size of the Black Flame Prison (Hell Flame).