One possible scenario is that Naruto is in a fight against a very strong enemy. He realizes that his normal attacks are not effective, so he taps into the black fire power. He then uses it to overpower his foe with its intense heat and destructive force.
One possibility in this fanfiction is that Naruto is exposed to a powerful and otherworldly energy source. This energy could be from a different dimension or a long - forgotten relic. When he comes into contact with it, his body absorbs and adapts to it in a way that allows him to control black fire. This new power also comes with its own set of challenges and responsibilities for Naruto as he tries to integrate it into his existing set of skills.
He might face powerful rogue ninjas. These ninjas could be after the power of the black fire for themselves. They would see Naruto as a threat and try to take him down to get the power.
There are fanfictions where Naruto uses the black fire after making a pact with a powerful entity. This entity could have granted him the ability to use the black fire. He may use it in a strategic way during battles, like surrounding his opponents with walls of black fire to trap them.
The plot could be that Naruto, during a mission in a strange land, comes across a cursed fire. This fire has the power to consume everything in its path. At first, Naruto struggles to control it, but with the help of his friends and his own determination, he learns to harness the power of the fire for good. He then uses it to protect the land he is in from a greater evil that was also after the power of the cursed fire.
Yes, there are likely fanfics out there where Naruto uses the Black Leg Style. Fanfic writers often like to combine different elements from various anime and manga. You can try searching on popular fanfic websites like FanFiction.net or Archive of Our Own.
In fanfiction, black fire bending can be seen as an exotic and powerful addition to the Naruto universe. It could be a result of some ancient and forgotten jutsu that has been rediscovered. For example, a hidden scroll might hold the secrets to black fire bending, and a curious ninja stumbles upon it. This new power then sets them apart from other ninjas, as black fire is often depicted as more dangerous and harder to control than regular fire in fanfiction.
One good example could be a story where a character from Naruto gains the ability to bend black fire. This could lead to unique battle scenarios. For instance, they might use it to counter the regular fire jutsus in the Naruto universe in a more powerful and unexpected way. Another example could involve a crossover where a Naruto character teams up with an Avatar: The Last Airbender character who is also a black fire bender. They could combine their skills to face a common enemy.
In fanfiction, anything can happen. The author might have created a crossover concept where Naruto's abilities are combined with elements from Highschool DxD. Maybe it's a result of some magical or scientific experiment in the fictional world of the fanfic. It could also be a unique power that Naruto acquires during his adventures in the Highschool DxD universe.
It could be used for combat. The black fire might have properties like burning through magical barriers or causing extra damage to supernatural beings in the Highschool DxD world. For example, it could be really effective against devils or fallen angels.