Well, it could mean that Percy is about to face some really dark and powerful forces. Since Frostmourne is a very significant and often evil - associated item in its original context. Maybe Percy will struggle with the corrupting influence of the sword.
He might become more brooding. Frostmourne has a dark and gloomy aura, so Percy could start to withdraw into himself, constantly fighting the sword's negative influence on his mind.
Naruto could be initially attracted to Frostmourne's power. He might touch it and start to feel a strange connection, but then quickly realizes the danger it poses.
One common trait is enhanced combat skills. Since he is the God of War, he would be expected to be an even more formidable fighter. Another trait could be a more aggressive and assertive personality, as war often demands such qualities.
Well, he could become corrupted. Frostmourne has a history of corrupting those who wield it. Anduin, being a paladin, his struggle between the light and the dark could be a central theme. Maybe he starts to question his beliefs in the light.
Perhaps Naruto could be transported to the Warcraft universe where Frostmourne exists. He teams up with the heroes there to stop Arthas, and along the way, he discovers new chakra - like powers related to the Frostmourne's magic.
Often, the fanfictions play with the idea of power. Frostmourne is a very powerful and somewhat evil object. In these fanfictions, it can either corrupt or be used for good in the Harry Potter universe. For example, Harry might have to resist its allure. It creates a lot of internal and external conflict. Another unique aspect is the cultural clash. The wizarding culture in Harry Potter is different from the more dark and gritty world related to Frostmourne. This clash gives room for unique character interactions and plot developments.
One common type is enhanced combat skills. It's like he has the reflexes and strategic thinking of a high - level gamer in battles. So he can anticipate his opponents' moves better. Another is the ability to 'hack' or manipulate the magical systems in the Percy Jackson universe as if they were game glitches. This gives him an edge in situations where normal rules don't seem to apply.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3). There are often a plethora of Warcraft - related fanfictions there. Just use the search bar and type in 'Anduin Gets Frostmourne' and it should bring up relevant stories.
One common ability is enhanced water manipulation. Since he's still related to Poseidon, his control over water becomes supercharged. He could do things like create massive tidal waves with just a thought. Another ability is the power to manipulate chaos energy. This might be used to distort reality around him, making it difficult for enemies to attack him directly.