One way this could happen is if there is a prophecy in Naruto's world that is misinterpreted. The prophecy might lead Naruto to seek out power in a different dimension, which turns out to be Vergil's. As he tries to adapt to this new and dangerous world, he starts to take on Vergil's characteristics. He could learn Vergil's swordsmanship and combine it with his own ninja techniques, creating a whole new fighting style.
In this fanfiction, perhaps Naruto is sent to Vergil's world for some reason. There, he might be exposed to the same kind of power sources as Vergil, like demonic energy. His own chakra could start to mutate and transform, gradually making him more like Vergil. And with Naruto's strong will and Vergil's combat skills, it would create a really interesting character.
Well, one way could be through some sort of dark magic crossover. Maybe a powerful dark wizard in the Naruto universe decides to experiment and accidentally fuses Naruto's chakra with Sauron's essence. Another possibility is that a strange interdimensional rift opens and Naruto is pulled into Middle - earth and somehow gets corrupted by Sauron's power and gradually transforms into him.
One way could be through some sort of dark magic or a powerful curse. Maybe Naruto is tricked into performing a ritual that binds him to Lucifer's essence. Another possibility is that in a strange alternate dimension, Naruto's chakra is corrupted and mutates him into a being similar to Lucifer.
Well, one possibility could be that Naruto inherits Sparda's incredible demonic powers. He might use these new powers to protect Konoha in ways he couldn't before. For example, his combat skills could be enhanced to a whole new level, allowing him to defeat enemies with ease. Also, his relationship with his friends might change as they try to understand this new side of him.
The plot could also involve Naruto being transported to the world of Sparda. There, he has to learn the ways of that world to survive. He might meet characters similar to those in his own world, but with different abilities. And he could form new alliances, like teaming up with demons who oppose the main evil forces in the Sparda universe.
One way this could happen is through some sort of magical or demonic interference. Maybe a powerful dark magic spell was cast that merged Naruto's essence with Alucard's. Another possibility is that Naruto, in his quest for more power to protect his village, unknowingly delved into some ancient and dangerous knowledge that led to this transformation.
It could also be that a villain from the Shazam universe tries to take over Naruto's world. In the process, they create a situation where Naruto's chakra system gets infused with the magic of Shazam. This would be a really interesting development as Naruto would have to figure out how to use his new powers in a way that doesn't conflict with his ninja training. He might use his new - found flight ability to quickly get to different battle locations and his super strength to overpower enemies that were previously difficult to defeat.
One way this could happen is through some sort of powerful and dark magic or forbidden jutsu. Maybe there's a powerful villain who has the ability to transform Naruto into an Arrancar against his will. This would completely change Naruto's powers and his place in the ninja world. He would likely gain the powers of an Arrancar such as the ability to use Cero and Sonido, which would make him extremely powerful and also an outcast among his former ninja comrades.
Well, one way could be through some sort of strange magical or scientific experiment. Maybe a rogue ninja or a mad scientist creates a device that accidentally fuses Naruto's chakra with Frieza's essence. This would start to change Naruto's physical and mental state, gradually making him more like Frieza.
Well, it could start with Naruto being exposed to some dark chakra that warps his mind, making him adopt Bane's ruthless and power - hungry ways. Maybe he loses all his friends in a tragic event and turns to Bane - like methods to gain enough power to prevent such a thing from ever happening again.
In a fanfiction, it might be due to a curse. A powerful curse placed on Naruto by an unknown enemy. This curse could be designed to break down his identity and replace it with that of Sephiroth. As the curse takes effect, Naruto would first start to experience strange dreams and visions of Sephiroth's life. Then, his physical abilities would change, becoming more in line with Sephiroth's superhuman strength and speed. His chakra might even transform into something more like Sephiroth's magic power, and his mind would be filled with Sephiroth - like thoughts of power and domination.