Perhaps it's a unique staff. A staff could have different uses, like being a conduit for chakra. It could also have hidden compartments or be made from a rare material that gives Naruto an edge in battles. For example, it might be made from a metal that can absorb chakra attacks and then redirect them.
Well, it could be a sword with the power to cut through any jutsu. Maybe it has some special markings that only Naruto can understand. It might also be a staff that can control the elements in a unique way, giving Naruto an edge in battles.
If the weapon is a chakra - enhancing tool, it will boost Naruto's existing jutsu. For example, his Rasengan could become more powerful, being able to break through stronger defenses.
His friends might be a bit wary of the sentient weapon at first. Sasuke, for instance, might be suspicious and think it could be a danger. But Naruto would try to convince them that it's okay. Over time, if the weapon proves useful, they could come to accept it.
The shadow weapon might be an ancient relic from Wave. Maybe it was created by a powerful shinobi from Wave long ago and has been lost until Naruto found it.
Well, it could be a key to unlocking new powers for Naruto. Maybe it has some special chakra - enhancing properties that help him in battles against stronger foes.
It might be a set of shuriken, but not the ordinary ones. These shuriken could be larger and have a unique shape, like the outline of a wave. They could also be made of a special metal that's resistant to most forms of damage and has the ability to return to Naruto's hand like a boomerang.
It could also change his stealth abilities. Since it's a shadow weapon, it may allow Naruto to blend into shadows more easily, making it easier for him to spy on enemies or launch surprise attacks. This would be a great advantage in his ninja missions.
One possibility is that the sentient weapon has its own goals and motives. It could lead Naruto on a quest to fulfill something it desires. For example, it might need to be reunited with its original creator or find a special place. Naruto would likely go along with it, seeing it as an adventure and also because he trusts his instincts about the weapon not being evil right away.
With his mentor figures like Kakashi, they would likely try to guide Naruto on how to handle this new discovery. They would caution him about the potential dangers and help him understand the responsibilities that come with possessing such an ancient weapon.