Macguffin or interesting plot thread?

Eh, I don't think you need to stress over it at this point. Write the story you enjoy most, and don't fear if people hate it. You can always go back after the beta reading stage.

 

Ah, yes, I know the struggle. I've rewritten the same scene about 12 times and I'm still not quite confident with it. :/ Something you could do is flesh out a chapter with the new material that popped up in your head and then post it in critique to get a feel for how it reads.

 

If that is all the merchant is, then he probably is a Macguffin. Nothing wrong with that of course. Plenty of good books out there with great Macguffins.

If you want him to be more, then the merchant needs to play a bigger role in the story. There needs to be a point to him besides showing the antagonist. Maybe he's related to one of the protagonists. Or one of them falls in love with his daughter (or all three do...). Or something else. There needs to be more point to him than just getting it started.

 

It's funny but I always think of a MacGuffin as an object or goal. Your merchant, if you intend for him to reappear throughout the story, SHOULD have some tie to other, main characters (the protagonists and/or antagonists). That can develop through the telling of the story as with Prince of Spires suggestions above and doesn't have to be there at the start, but he can also be there simply to help show readers the situation/special talents/purpose etc of the main characters/protagonists as the story opens. Maybe one is a healer or one recognizes the value of something in the merchants possession or one knows Lich lore and recognizes a mark on the skin or set a look in the merchant's eyes that hints at the darker tale to come.

A good example is the first chapter of VE Schwab's "A Darker Shade of Magic", in which the secondary character of the mad King is never seen again but perfectly allows the reader to establish a great deal about the visiting main character, the magic he wields, his empathy and the unusual world the story takes place in. For my money it's the perfect example of the "throwaway character" because it reveals so much in a few pages and the throwaway DOES have ties to the rest of the world. The chapter also puts an object the main character possesses, a coat, front and center, the strangeness of which will never be fully explained in the book, but that's ok too!

But here is a tip. That first chapter could only be written once Schwab knew the entire story and she outlines her stories from the ending back. There are promises made that pay off much later. So my suggestion is to jot down your opening ideas, maybe sketch out a rough draft of the opening and then set it aside to revisit over time. Don't get hung up on trying to create the perfect beginning because, until you know the whole story and understand what you want/need to seed into those opening pages, you'll be hard pressed to really fit all you need to within them.

Best of luck!

 

If you want a character to introduce your protagonists to the antagonist, why not make them a henchman of the antagonist? They could still fake being unconscious and be part of the trap to take down the protagonists. Or be mind, controlled as you suggested at first, which would give an awesome feeling of danger and betrayal when the party is attacked by them.

The reason I suggest this is that a one-note character that only exists to give exposition might be boring for a reader. It doesn't have to be, of course, but the chances of them being uninteresting are I think quite high. You could also go in another direction and have them be an interesting, perhaps even quirky, recurring character whose presence has greater implications on the plot as a whole.