I'm not sure specifically what 'chalice novel' is about as it could be a very specific or newly created concept. It might be a novel that has a chalice as an important symbol, perhaps related to a mystery, a historical event, or a fantasy story.
One aspect of 'Merlin the Poisoned Chalice' fanfiction could be the exploration of Merlin's character in this particular scenario. It might show his inner strength and loyalty as he faces the danger of the poisoned chalice.
Since I don't know the specific 'chalice novel', I can't tell you the main characters. It could be a hero on a quest for the chalice, a villain trying to steal it, or a group of friends involved in the chalice - related mystery.
I'm not entirely sure without reading it, but usually there would be a protagonist who is likely to be closely associated with the silver chalice. Maybe it's a young adventurer.
The main theme could be about the significance of the silver chalice. Maybe it's a symbol of power, or it has some religious or historical importance in the story.
Bloodborne's story is set in a gothic horror world full of beasts and dark secrets. You play as a hunter trying to uncover the mysteries and survive the nightmarish encounters.
Fanfiction in this area can be unique because it allows for different interpretations of the characters' emotions. For example, it can show Merlin's fear in a more detailed way when he's faced with the poisoned chalice, rather than just a brief moment in the original. Also, it can give other characters like Gwen or Morgana different roles in the story related to this event.
There are quite a few out there. One popular one might be 'The Adventures of Cuphead and Chalice' which has an interesting plot where they go on a journey to retrieve a lost artifact in the Inkwell Isles. It has great character development for both of them.