At first, it would likely be very strained. Harry has a history of conflict with the Malfoys, and Narcissa is a Malfoy. But over time, they might start to communicate more. They could find that they both have a love for the wizarding world's history and start sharing stories. This could lead to a more amicable relationship.
I'm not sure if there is such a specific fanfic available. Fanfics cover a wide range of scenarios, but this one might be quite rare or not commonly known. You could try searching on major fanfic websites like Archive of Our Own or Fanfiction.net, but it's not guaranteed that you'll find it.
One good fem Harry Potter Narcissa fanfic could be 'The Hidden Bond'. It explores a different dynamic between a female Harry Potter and Narcissa. In this fic, their relationship starts off as a mystery but gradually unfolds into a complex bond that affects the entire wizarding world around them.
In some fem Harry Potter fanfics, Narcissa can become more of a protector. For instance, if the story involves a female Harry Potter who is in danger, Narcissa might step up in ways she didn't in the original series. She could be more assertive in using her influence within the Death Eater circles to safeguard Harry.
Draco Malfoy is often a very popular character in these fanfictions. Since Narcissa is his mother, his relationship with her and how it is affected by her interactions with Harry can be a major plot point.
Narcissa might be very angry with Harry's abusers. If it's the Dursleys, she could use magic to give them a taste of their own medicine, but in a more subtle way so as not to draw too much attention. For example, she might make their houseplants grow out of control or cause their food to spoil every time they're mean to Harry.
In some fanfictions, Harry can be written as Narcissa's son. Fanfiction allows for creative reinterpretations of the original story. Writers might do this to explore different character dynamics or create unique plotlines.
There are some out there. However, the idea of a Harry Narcissa romance is quite an interesting and somewhat controversial concept in the fanfiction world. Some stories might explore a more platonic or unrequited form of love, while others might go for a full - blown romantic relationship. It really depends on the writer's creativity.
In Harry and Narcissa fanfiction, the theme of trust is quite common. They have to build trust in each other, which is not easy given their different allegiances. Another common theme is protection. Narcissa may want to protect Harry for some reason, or vice versa. And the theme of changing loyalties is also present. Their relationship can cause them to question and even change their loyalty to their respective sides in the wizarding conflict.
Most platforms have strict rules against explicit and inappropriate content in fanfics. So, it's highly unlikely that you'll find a fanfic with such a title or theme in a legitimate place.