Read The Muggle Death Eater and His Daughter by Gr8t_78 here: https://archiveofourown.org/works/58788139/

Summary:

The muggle man was browsing the stacks at Flourish and Blotts when Lucius spotted something that would change the course of history.

The muggle had a Dark Mark.

Surely this man– the father of a mudblood, Lucius reminded himself– hadn't been a Death Eater?

Lucius didn't know how it was possible, but he vowed there and then that he and his family would do everything in their power to protect this man's daughter-- their very lives could depend upon it.