Perhaps a rogue ninja who has delved too deep into dark jutsu. This ninja could be jealous of Naruto's power and wants to prove that his dark powers are superior. So, he becomes the main antagonist, forcing Naruto to explore his own ability to control the darkness in order to defeat him. The rogue ninja might have some connection to Naruto's past, like being a former student of one of the Hidden Leaf's dark - arts researchers.
It could be an ancient evil that has been lurking in the shadows for ages. Maybe a dark spirit that was sealed away long ago and is now breaking free, and Naruto has to control the darkness within himself to fight it.
There could be a new organization in the fanfiction that worships the power of darkness. Their leader, who is extremely powerful and manipulative, sees Naruto as a threat to their plans. This leader could be the main antagonist. He might try to corrupt Naruto or simply eliminate him. As Naruto tries to control the darkness within himself, he has to face this organization and its leader, who has a deep understanding of the dark arts and uses it to his advantage.
A powerful dark spirit might be the main antagonist. This spirit could sense Naruto's new ability to control darkness and see him as a threat. So it starts causing chaos in the ninja world, forcing Naruto to use his dark - controlling powers to stop it. The spirit could have the ability to possess other ninjas, making them do its bidding.
There could be a powerful shinobi from within Konoha who is jealous of Naruto's unique ability to control snow. This shinobi might secretly work against Naruto, trying to undermine his progress and steal the power for himself.
It could be Orochimaru. He's always been interested in new and powerful jutsus. He might see Naruto's shadow - controlling ability as something he wants to steal or use for his own evil purposes.
It could be the Hidden Leaf Village itself. If the village turns against Naruto through false accusations or unjust treatment, it becomes the force that drives him to the dark side. The elders or some of the more conservative ninja in the village who oppose Naruto's unorthodox ways could be the face of this antagonism.
It could be an old ninja who was once banished from Konoha for using dark jutsu and now wants revenge. He might see Naruto as a symbol of the village he hates and tries to corrupt him by luring him into the darkness.
It could be Orochimaru. He has always been interested in power and has no qualms about using dark methods. He might see Naruto as a perfect vessel or a tool to further his own dark plans, and through his manipulations, Naruto starts to be consumed by darkness.
It could be a rogue ninja from within Konoha who has embraced the 'darkness' fully and wants to reshape the village according to his own twisted ideals.
In 'naruto controls the darkness' fanfiction, it could also be that Naruto is exposed to a dark energy source accidentally. This energy starts to influence his thoughts and actions in negative ways at first. But being Naruto, he doesn't give in. He uses his strong willpower and the positive relationships he has with his friends like Sasuke, Sakura, and others to find a way to control this new dark power. He might have to face some of his own demons from his past, like the loneliness he felt as a child or the times he failed to protect his friends, and use these experiences to better understand and control the darkness within him.
In a 'naruto controls darkness fanfiction', perhaps Naruto initially struggles with this new power. He's a hero associated with light and positive energy, so controlling darkness could be seen as a corruption. But then he realizes he can use it for good, like infiltrating dark organizations by blending in with their dark - themed jutsus.
It could be a rogue Huntsman who wants to use Ruby's power to control Grimm for his own evil purposes. He might believe that with the Grimm under his control, he can rule over Remnant. He would then be constantly trying to kidnap Ruby or figure out the secret behind her power, and would oppose team RWBY at every turn.