It is a composite skill of Area Boundary, Fire Manipulation, and Black Lightning. First, space is secured with Area Boundary, he then accelerates the molecular motion inside with Fire Manipulation. So it will produce high temperatures. Finally, the magical power inside the space acts as fuel, and the Black Lightning turns it into a 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. Its 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.
The number and size of the fireballs created always depends on the magical ability and magicule amount of the user. Benimaru, for example, was seen creating six big ones in the anime against the Orc Disaster's subordinates, killing thousands of orcs in the process. However, the Orc Disaster himself was able to survive a direct hit with one due to his fire resistance.
With the support of Great Sage, it's possible to reduce the size of the Black Flame Prison (Hell Flame).