Mercy Seat (The) by Bonetree

Months after the events of "City of Light," Mulder and Scully investigate a haunting by an entity that stalks a troubled woman and her husband. But what is this thing that haunts her? And haunts us all?

ARCHIVIST'S NOTE: the AO3 version doesn't include the prologue, but it's available at the local link.

AUTHOR'S NOTE: This story takes place in what I call the Goshen Universe, an AU started with the story "Goshen" and which continues through "Secret World" and "City of Light." That said, you don't have to read the previous three stories to understand this one, since the plot sort of starts over with this story. However, there will be references to events from all three stories through this one, and original characters from those stories whom it will be assumed you are already familiar with, so you might start wondering to yourself what I'm referring to if you haven't read the other stories. This is also an established MSR story, and the previous three stories detail the beginning and evolution of the relationship, which might be helpful for you to know.