There could be a new, powerful dark wizard or witch who has emerged. This character might not be directly related to Voldemort but has their own reasons for wanting to bind Harry. They could see Harry as a threat to their own rise to power in the wizarding world, so they take this extreme step to incapacitate him.
Yes, there are quite a few. You can start by looking on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Harry Potter Binds Demons' in their search bars, and you'll likely find a variety of stories with different plotlines and interpretations.
Maybe there's a demon lord who has some sort of claim over the succubus. This demon lord sees Harry's binding of the succubus as an insult and a threat, and thus becomes the main antagonist. He would then send his minions or use dark magic to try and break the bond between Harry and the succubus.
One common theme could be the struggle between good and evil. Since demons are often associated with evil, Harry and his friends would be fighting to keep the demons in check. Another theme might be the discovery of new magic. Binding demons is not a common practice in the original Harry Potter books, so exploring this new type of magic could be a big part of the stories.
One possible plot could be that Harry, perhaps in a search for power or knowledge, binds the succubus. But then he has to constantly battle her wiles and temptations as she tries to break free. He might use magic he's learned at Hogwarts to keep her in check.
Well, it could be that Harry, in this fanfic, discovers a way to bind the succubus familiar through some ancient magic he stumbles upon. Maybe he needs its powers for a particular task, like fighting a new dark force in the wizarding world.
Harry could also be angry. Despite understanding Dumbledore's generally good intentions, being tied to a bed against his will is a violation of his freedom. He might struggle against the restraints and demand to be released immediately, especially if he feels that Dumbledore is overstepping his boundaries.
In some fanfictions, it might be a more symbolic act. For example, if a character is getting on the bed, it could signify a retreat from the chaos of the wizarding world outside. It could be a place where they feel safe and can think about their next moves, like Harry might do when he's worried about Voldemort and needs some quiet time to strategize.
The 'angel' could be a new character created by the fanfiction author. This character might have some extraordinary powers or a very kind heart that earns them the title of 'angel'. It could also be an existing character who is portrayed in a new light, like Luna Lovegood. Luna has this ethereal quality about her already, and in this fanfiction, she could be elevated to an 'angel' - like status, perhaps being the one who always saves the day with her unique insights.
She could be an original character created by the fanfiction author. Maybe a witch who was secretly trained by a dark wizard but has a change of heart when she meets Harry.
Perhaps his grandparents were great adventurers in their time. They could have traveled to far - off magical lands and encountered strange and wonderful things. Their experiences could be passed on to Harry, inspiring him to explore more of the wizarding world and its secrets. They might also have been involved in some great battles in the past, and their war stories could give Harry valuable lessons for his own fight against Voldemort.