There's a high chance he'll encounter Kreacher. Given Kreacher's loyalty to the Black family and his initial dislike for Harry, their meeting could be quite tense, especially when Harry is already angry. Kreacher might even try to play some tricks on Harry in this state.
He will most likely meet Sirius Black. Sirius is an important figure in Harry's life and his presence at Grimmauld Place is quite significant. They could have a deep conversation about Harry's anger and what to do next.
Well, perhaps Harry is angry because of some injustice he faced at Hogwarts. When he arrives at Grimmauld Place, he might find some secrets about the Order of the Phoenix that further fuel his anger. He could also have an argument with some of the residents there, like Sirius or Mrs. Weasley.
Well, in fanfiction, it could be that when Harry arrives at Grimmauld Place, he discovers new secrets about the Black family. There could be hidden rooms or magical artifacts that are crucial to the fight against Voldemort. Maybe he stumbles upon a diary of one of the Black ancestors that reveals a long - forgotten spell or prophecy.
Dumbledore might make an appearance. He could come to give Harry some advice or to share some important information about the war against Voldemort. Dumbledore might also be interested in seeing how Harry is faring in the old Black family house and if he's discovered anything of significance there.
It could be that there is no traditional 'antagonist' in the sense. Maybe it's just the situation and the conflict between Harry and Ron's personalities. But if we had to consider one, it could be an outside force that is influencing Ron's actions and making Harry distrust him. For example, a new dark magic object that is affecting Ron's judgment.
There's a possibility that after Sirius's death, Harry just sort of took over. He had already spent time there and knew the place well. The remaining members of the Order of the Phoenix, seeing his connection to the place and his leadership potential, supported his claim to Grimmauld Place.
One key event could be Harry exploring the secrets of Grimmauld Place. He might discover hidden rooms or artifacts. Another could be him hosting Order of the Phoenix meetings there, which would involve a lot of strategic discussions and planning against Voldemort.
Since I haven't read this fanfiction, it's difficult to give a detailed account. However, it may be a story that uses Grimmauld Place as a backdrop for some sort of romantic or sexual plot. It could be about how the characters interact with each other in this location in a more private and personal way. It might also explore the secrecy and the unique atmosphere of Grimmauld Place and how that impacts the relationships and the sexual or romantic elements within the fanfiction.
In some fanfictions, his arrival is a chaotic scene. There are people running around, trying to secure the place and keep Harry hidden. In others, it's a more solemn moment, with everyone waiting in silence for his arrival, aware of the great burden he carries.
Well, the plot could involve Harry exploring the secrets of Grimmauld Place on his own. Maybe he discovers hidden rooms filled with old Black family magic items. He might also struggle with feelings of loneliness and isolation, but at the same time, use the solitude to practice his magic without distractions.
Harry Potter is often a main character. Since he has a connection to the place through Sirius Black, he's frequently involved in the stories set in 12 Grimmauld Place. His experiences and growth as a wizard can be explored in the context of this spooky old house.