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.
In a more complex scenario, Anduin getting Frostmourne could be part of a bigger plan by an ancient evil. He could be a pawn in a game that spans across different realms. His journey with Frostmourne could lead to unexpected alliances and betrayals as he tries to come to terms with the power he now wields and the destiny that seems to befall him.
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.
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.
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.
Perhaps she gets hurt while trying to protect someone she cares about. She could be shielding a friend from an attacker's blow, and in the process, take the brunt of the injury herself. This would then set up a story of her recovery and the revenge she might seek on the one who caused the harm.
Well, in fanfiction when Saitama gets drunk, he may interact with other characters in a different way. For example, he could become more talkative and engage in long conversations with villains or heroes alike. This could lead to some unexpected alliances or rivalries. Also, his physical abilities might be affected in strange ways. Maybe he would still be strong but lack the precision he usually has, resulting in some clumsy but still powerful actions.
In a 'naruto gets captured' fanfiction, he might be tortured for information. His captors could be after the secrets of his Nine - Tails chakra. They may try to use him as a bargaining chip against Konoha.
In a 'l gets kidnapped fanfiction', there could be a lot of drama. Maybe his friends would start a search mission right away. They might follow some small clues like a piece of cloth from his clothes left at the scene.
Well, in the 'sonic gets a makeover fanfiction', Sonic could be given a makeover by his friends. They might change his color scheme a bit, like adding some blue highlights to his fur. His speed could also be enhanced in a new and exciting way as part of the makeover. Maybe he gets some new power - up items that are integrated into his new look.
In some fanfictions, Dumbledore is portrayed as having a lot of secrets. If he gets caught, it could be related to his past. Maybe some old enemies find out where he is and catch him. Then he would have to deal with his past coming back to haunt him. His friends might try to rescue him, but it would be a very difficult situation for him, and it could change the whole dynamic of the fanfiction story.
In the 'tohru gets sick fanfiction', perhaps her friends would be very worried about her. They might take turns to take care of her, bringing her warm soup and making sure she takes her medicine on time.