Bellatrix didn't die during the final battle. Harry finds her body and wants to punish her for all the pain she caused him. He brings her to the forest and ties her up and let's the thestrals and unicorns have their way with her.
Harry needs heirs to fill his family seats. He thinks Narcissa is the perfect person for the job. She will give him all kids in one shot with the help of Black Sex magic and potions.
Harry creates an mc "magical club" and his group goes to war with Voldemort. He does things that Dumbledore refuses to do kill.. pm me for more details of you are interested.
Harry rescues Narcissa from Azkaban but is that a good thing? Harry has a way to make sure she learns her lesson. This is dark so be warned lemons and smut hardcore Harry/Narcissa trigger warning.