Maybe Harry is in a dire situation where he needs a powerful source of magic to defeat Voldemort, and Bellatrix's magic seems like a viable option.
I'm not sure if there is such a specific fanfic. You could try searching on major fanfic websites like Archive of Our Own or Fanfiction.net.
Well, in a 'Harry impregnates Bellatrix' fanfic, it might explore the chaos that would ensue in the wizarding world. Harry's friends would be horrified and confused. Bellatrix, being who she is, might have her own dark motives for allowing or even causing such a situation. It would be a very complex and disturbing story that would likely rewrite many established character traits and relationships in the 'Harry Potter' universe.
A plot might involve Harry being captured by Bellatrix. Instead of harming him immediately, she is intrigued by his power. She tries to break his spirit but Harry resists. Over time, through their power struggles and intense conversations, a strange connection forms between them that changes their relationship in unexpected ways.
One possible plot could be that Harry discovers this long - lost family connection after some mysterious event. Maybe he finds a hidden letter or an old family heirloom that reveals the truth. This discovery could throw him into an identity crisis as he has to come to terms with being related to a Death Eater. He might then try to find out more about his family's past and his connection to Bellatrix, which could lead him into dangerous situations as he delves deeper into the dark secrets of his family.
She becomes more empathetic. Usually known for her cruelty, in this fanfic she starts to care for Harry, which means she has to let go of some of her cold - heartedness. Her priorities shift from just following Voldemort blindly to protecting Harry.
In this fanfiction, it might be that Snape is forced to look after Harry due to some sort of emergency situation at Hogwarts. The act of feeding Harry a bottle could be a turning point in their relationship, leading to Snape seeing Harry more as an innocent child rather than just James Potter's son, and Harry starting to trust Snape a bit more.
One possible plot could be that Harry discovers a powerful and dark magic spell that allows him to gain control over Bellatrix. He then uses this to his advantage to gather information about Voldemort's plans from her. Harry might struggle with the moral implications of enslaving someone, even a Death Eater like Bellatrix.
One possible plot could be that Bellatrix, in a strange turn of events, decides to train Harry in the Dark Arts not to turn him evil but to make him a more formidable opponent against Voldemort. She might see his potential and think that by teaching him some of the Dark Arts, he can better understand Voldemort's tactics.
If such a fanfic were to exist, perhaps it could involve Harry somehow using a new and powerful magic 'force' to subdue Bellatrix and then there could be a strange turn of events where they end up in a situation that seems like they are fuxking. But this is all very outlandish and not in line with the general tone of the original Harry Potter series.
Well, perhaps Bellatrix was under some sort of spell or coercion when she had Harry. In the fanfiction, Harry might start having strange visions or powers related to Bellatrix's dark magic. As he enters Hogwarts, he is constantly suspected by Dumbledore and the Order of the Phoenix because of his connection to Bellatrix. But Harry, being Harry, still wants to fight against Voldemort, which creates a complex web of relationships and power struggles within the wizarding world. He may also try to find out more about his mother's past and what really happened when he was born.