Perhaps Harry gains control because of his role in defeating Voldemort. In the wizarding world, power often shifts after great battles. His status as the 'Boy Who Lived' might give him certain rights or abilities to influence families like the Blacks. He could use his new - found influence and respect in the wizarding community to take charge of the Black family's situation.
Well, it could be through some sort of magical inheritance. Maybe he discovers a long - lost spell or charm that gives him power over the Black family's affairs. It might also be related to his connection with Sirius Black. Since Sirius was part of the Black family and Harry was so close to him, there could be some kind of transfer of influence.
There could be a plot point about Harry's relationship with the Black family's house - elf or other magical creatures associated with the family. He might have to deal with their loyalty issues. And perhaps there are other external forces that try to use Harry's control over the Black family for their own purposes. This could lead to a series of conflicts and challenges for Harry as he tries to keep the Black family under his control while also protecting them from outside threats.
In some fanfictions, Harry might gain the Black family lordship through a complex web of magical inheritances. For example, if there are unforeseen loopholes in the family's magic - bound inheritance laws. Maybe a magical will is discovered that names Harry as the rightful heir due to his relation to a long - forgotten Black family member.
Well, in some fanfictions, Harry might gain control over the Wizengamot through his heroic deeds. For example, if he saves the wizarding world from a great threat again, the public support could be so strong that he can influence the Wizengamot's decisions. He could also use his knowledge of magic, especially some ancient or powerful magic that gives him an edge in the political arena of the wizarding world.
One possible plot could be that Harry discovers some ancient magic that gives him the power to control the Black family Grim. He might use this power to uncover secrets within the Black family that were long hidden. Maybe he uses it to protect the family from dark forces that are after their power or knowledge related to the Grim.
There's also the possibility that an ancient dark magic entity, which has some connection to the Grim, is the main antagonist. It tries to manipulate Harry into using the Grim's power in a way that will free it from its ancient prison or some such situation, and Harry has to resist this while still maintaining control over the Grim.
In some fanfictions, Harry might gain the ability to control Fiendfyre through intense magical training. He could be mentored by a powerful wizard who teaches him the dark arts in a controlled way, allowing him to master Fiendfyre. Another possibility is that he inherits some ancient magic that gives him this power. For example, if he discovers a long - lost family heirloom that has the magic to control such dangerous spells.
Well, it could be that Naruto's own chakra nature evolves in an extreme way. His Uzumaki bloodline might have some hidden potential that, when combined with his experiences and growth as a ninja, mutates into the ability to control black holes. For example, his chakra, which is already very powerful, could reach a new level where it can warp space to form black holes.
In some fanfictions, it could be through intense magical training and study. Maybe he discovers an ancient spell or a hidden magical technique that allows him to assert control over the dangerous Fiendfire.
Well, it might be related to his connection with the Dark Lord. Since Voldemort was able to control the basilisk before, and Harry has a unique connection to Voldemort through the scar. This connection could be manipulated in some way in the fanfiction, allowing Harry to take over the control of the basilisk from Voldemort.
One great 'Harry Potter Black Family Fanfic' is 'The Noble and Most Ancient House of Black' which delves deep into the family's history. It shows how the family's dark past and their traditions influenced the events in the wizarding world. Another one could be 'Black Family Redemption' that focuses on some of the Black family members trying to break free from the dark legacies they've been burdened with. And 'Sirius's Secret Heir' is also interesting as it explores a possible hidden heir within the Black family.