Disclaimer: I am not J.K. Rowling. I do not own Harry Potter.
Note: The following near drabble assumes an alternate universe of some kind. It is identified as 'mystery' and is rated as 'T'.
She does it out of desperation. Because everyone who was supposed to be alive and a friend at this point is dead. Or in prison. Or turned traitor. Or 'being watched and held accountable'. Or something likely much, much worse.
She does it because she has this animal instinct, of how things were somehow supposed to otherwise be; that she was meant to become and to be a mother; that it is meant to somehow be important.
And she does it with him because out of everyone she knows and cares about who might otherwise have been a father there is nobody whatsoever available left, and out of everyone else there is, realistically, only him. The Enemy.
And nine months later, she gives birth, and looking down at her daughter, Persephone Circe Black, Narcissa knows that one day all will once again be right with the world. The ascendancy of the tyrants will not endure, Tom Riddle will return and make all well, and the blood of her darling James will be avenged ten thousandfold…
The piece is identified as 'mystery' as indicated in the opening post, and it's a mystery to which I don't have many of the answers - although perhaps in this particular alternate universe 'the tyrants' might have first names of 'Albus' and 'Gellert'.
The protagonist was originally intended to be someone else, but in the process of writing ended up morphing into Narcissa, because that felt so much more interesting in trying to go outside the canon box. The date is probably some time in the late 1970's or early 1980's, after 'the tyrants' have just been engaged in some sort of war or purge to tighten their grips on power.