One possible antagonist could be Orochimaru. He might be interested in Naruto's new Shadowman powers and try to steal them for his own evil purposes.
Another antagonist could be a new character created specifically for this fanfiction. This character could be from a clan that has a long - standing feud with the power of the Shadowman. They see Naruto as an abomination for having these powers and will stop at nothing to destroy him. They might have unique abilities that are specifically designed to counter Naruto's shadow - based powers, such as the ability to create light that can dispel shadows.
The Akatsuki members could also be antagonists. They would see Naruto as an even greater threat now that he has the powers of the Shadowman. They might team up to try and take him down. For example, Sasori could use his puppet techniques against Naruto's new shadow abilities, while Deidara might try to blow him up with his explosive jutsus. Their combined efforts would pose a serious challenge to Naruto.
There could be other ninja villages that see Naruto's new powers as a threat. They might form alliances to oppose him, fearing that he will use his god - like abilities to dominate them. Also, there might be dark forces or ancient evils that awaken because of Naruto's transformation. These entities could oppose him as they try to maintain the balance of power in their own dark ways.
Well, perhaps Naruto's transformation into the Shadowman is the result of some strange experiment or a curse. He then has to struggle with controlling these new powers while still maintaining his identity as a ninja. He might use his new shadow powers to fight against enemies that are immune to normal ninja techniques, like some powerful dark entities. This would add a whole new dimension to his battles and his role in the Naruto universe.
One possible antagonist could be Kronos. Since Percy is becoming a Titan of Time, Kronos, who has his own history with time - related power plays, might see Percy as a threat and try to stop him. Another could be the gods themselves. They might not want a demigod to have such a powerful Titan - like status related to time.
There could be a rogue legendary Pokemon that sees Ash's transformation as a threat to the natural order. This legendary Pokemon, with its own set of motives, could be trying to prevent Ash from becoming a fully - fledged legendary - like being. It might set up various obstacles and battles for Ash, testing his will and strength at every turn.
The Hidden Leaf Village elders could be main antagonists. If Naruto turns dark because he feels betrayed by the village, they would be the ones representing the establishment he is against. Their conservative and sometimes self - serving ways could be the catalyst for Naruto's transformation, and then they would be the ones trying to stop him as he becomes a threat to their power and the village's traditional order.
It could be the old enemies who see Naruto's new form or role as a threat. Maybe Orochimaru, who is always after power, would try to stop Naruto or take advantage of his new state. He might think Naruto's power in this apocalyptic scenario can be harnessed for his own evil purposes.
The Hidden Leaf Village elders could be antagonists. If Naruto turns dark, they might see him as a threat to their power and try to stop him. Their actions against Naruto could further fuel his descent into darkness.
The Hidden Leaf Village itself could become the main antagonist. Since Naruto turns evil, the village that once nurtured him would now be against him. The village elders and some of the ninja who oppose Naruto's new evil ways would be in conflict with him. For example, Kakashi might be forced to fight against Naruto to protect the village.
One possible antagonist could be Orochimaru. He's always been interested in powerful and unique abilities. Naruto's new powers as the son of doomsday would be very appealing to him, and he might try to capture Naruto to study or steal his powers.
In this fanfiction, it's possible that an ancient and powerful beast is the antagonist. The beast might have captured Pyraa to lure Naruto into a trap. Naruto will need to use all of his skills, including his Sage Mode if necessary, to defeat this beast and save Pyraa.