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.
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.
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.
It might start with Spider - Man being exposed to a cursed artifact. This artifact could be related to the origin of Ghost Rider. For example, if it was an ancient amulet that once belonged to a Ghost Rider. Spider - Man, being curious as always, investigates it and accidentally activates its power. The dark energy within the amulet starts to change him, slowly turning him into a being with the powers and characteristics of Ghost Rider. His suit could start to change, taking on elements of Ghost Rider's flaming skull and chain - wielding abilities.
Well, it could be that Naruto is exposed to a powerful and corrupting force. This force warps his sense of justice. Deadshot's character represents a more morally gray area compared to Naruto's traditional view of right and wrong. Maybe Naruto has a series of traumatic experiences that make him question his own values. He then sees Deadshot as a model of someone who can get things done regardless of the cost, and thus begins to transform into someone like Deadshot.
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.
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.
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.
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, it could be due to a curse. There might exist a very ancient and powerful curse that was once sealed away. But through a series of events like tomb raiding by some evil forces, the curse was unleashed. Naruto, being in the wrong place at the wrong time, got affected by it and transformed into Shadow the Hedgehog.