According to Fox Mulder, was that - after World War I - Maurice and Lyda were "star-crossed lovers" who made a "lover's pact" and took their own lives. From that point on, they could be found haunting the house every Christmas Eve. Three double murders took place in the house over the following decades, leading Mulder to believe that the ghosts were involved.
Involvement with Dana Scully and Fox Mulder
At Christmas 1998, he and Dana Scully visited the house and encountered the ghosts. Scully learned that Lyda was indeed a ghost when she noticed the gaping entry and exit wounds through her torso. Maurice and Lyda in particular seemed to take great pleasure in terrifying Mulder and Scully.
After taking the illusion of Scullly, Mulder experiences a bullet wound to his chest after having been shot by Lyda, in the form of Scully, with Scully's firearm. In another room, Maurice takes of the illusionary form of Mulder and shoots Scully with her firearm in a similar fashion to how Lyda shot Mulder.
As the two agents are crawling to the front door, they eventually realize that Maurice and Lyda were playing tricks on them and their bullet wounds were not real they were never injured as the ghosts created illusion to make their wounds look and feel real as long as they believe in their situation. As Mulder and Scully left the house, Maurice and Lyda watched and claimed that Mulder and Scully were better than they thought as they escaped and the ghosts' plans were not fulfilled. Once the agents left, Lyda decided to spent Christmas with Maurice together as they have always done and always will.