Well, in order to write a continuation I'd like to ask you some questions to make sure that what is written is what you actually meant.

The countess was a woman with a grave secret many thought would be best left untold.

If MANY knew of her secret then it wasn’t really a secret, but a problem

For himself, he chose the Yellow Boy Winchester, a weapon that had saved the countess more than once before.

The Winchester had saved the Countess many times before. That could mean one a couple of things

a. There was a “beast” or “beasts” and George was able to scare them off by shooting from the gun.
b. There were several “beasts” of the same type and George was able to get rid of them using the Winchester. That actually implies that the “beats” were an actual animal because you cannot kill a werewolf or another none-life from a Winchester unless one uses silver bullets.

He knew she wouldn't have to use it, but it seemed to comfort her to have it in hand at times like these.

How could George know for sure that the Countess wouldn’t use the gun? Does he know it from experience? Maybe she doesn’t know how to use it?

1. Why would the “beast” come to the mansion during the storm? Can’t he, she or it arrives at the mansion at “regular business hours” so to speak?
2. Coming back to the question #1 If many people knew of the Countess’ problem, and no one did anything about it (like calling to the police) then the “beast” was somehow related to the Countess or was her responsibility.

Am I following your lead so far?