an evil dragon that was sealed away for 300 years

One possible reason could be that his godlike powers were seen as a threat. The village elders or some powerful forces might have feared that Naruto, with his immense strength, could lose control and cause destruction. So, they decided to seal him away to protect the village and the world at large.

In the novel "Mortal Cultivation Legend", Han Li sealed the evil corpse in chapter 1209. In this chapter, Han Li devoured the Nether Beast's demon core and successfully cultivated his second Nascent Soul. Then, he used his second Nascent Soul and the Profound Heavenslayer Sword to seal the Evil Corpse inside. While waiting for the TV series, he could also click on the link below to read the classic original work of " The Legend of Mortal Cultivation "!

I'm not entirely sure as I haven't read that specific fanfic. But it might be about Naruto being sealed away in some way, perhaps in a different scenario from the original story. Maybe some new villain or power manages to seal him.

In many fanfictions, when Naruto returns after being sealed away for years, the ninja world is often in chaos. His friends, now much older and perhaps more jaded, have to adjust to his presence. Naruto, on the other hand, has missed years of development and has to quickly catch up. His skills might be rusty, but his spirit remains strong. This can lead to him being underestimated by new enemies, but also provides an opportunity for him to show his growth in unexpected ways.

Maybe it's an ancient and powerful entity that has been awakened. Percy might be in the wrong place at the wrong time and this entity, seeing Percy as an intruder or a potential disruptor of its slumber, decides to seal him away using its otherworldly powers.

The story could also focus on the efforts to unseal Naruto. His friends and the characters from the crossover world could embark on a dangerous journey to find the key or the magic to break the seal. In a Naruto - Harry Potter crossover, Hermione might use her vast knowledge of magic to research a way to free Naruto from the seal.

One possibility is that the story could focus on how the village copes without Naruto. Maybe there would be a different kind of unrest or a search for a new way to deal with the Nine - Tails' power. Without Naruto, characters like Iruka might have a different role, perhaps becoming more involved in trying to find a solution to the power void.

Well, perhaps in this fanfiction, the Avatar enters the Naruto universe and causes some sort of imbalance. Naruto, seeing the chaos it might bring, decides to seal away the Avatar. As time passes, Naruto starts to learn more about the Avatar's world and realizes that there are good things about the Avatar too. So he begins a journey to find a way to unseal the Avatar without causing more harm, all while facing enemies who want to use the situation for their own gain.

You could try searching on fanfic websites like FanFiction.net or Archive of Our Own. Just type in 'naruto sealed away' in their search bars and see if it comes up.

Perhaps Percy discovers a secret that is so dangerous that the powers - that - be decide to seal him away to keep it safe. It could involve a prophecy that he might fulfill in a way that would change the world in a way they don't want. So, they use some ancient magic or a powerful artifact to seal him. For example, if there's a prophecy about Percy uniting two warring factions in a way that would upset the current balance of power, those in power might try to stop it by sealing him away.