Mythology - how much is enough and how much too much

Anyone who has read Tolkien knows that sometimes the background takes over. What is referred to in the Lord of the Rings films as "The Watchtower of Amun Sul" and then never explained, is given paragraphs of back-story in the actual novel Fellowship of the Ring. At the end of Return of the King, Tolkien was forced to add an appendix that was nearly 1/3 the length of the entire book in order to properly flesh out the background of things that had happened in the story. Every person, every town and every tree had a back-story in Tolkiens works. He died still working on his bible to Middle Earth, the Silmarillion.

So the question has to be asked; in writing a fantasy novel, how much mythology and background lore should I create, and how much of that should I include?

When I first started brainstorming this story, I basically wanted to do a "sequel" to Lord of the Rings. I intended to take the whole of both Fellowship of the Ring and The Two Towers as canon, and then devise a point somewhere in Return of the King where the worlds diverged. Somewhere along the way evil won.

That was all the background I intended to give that wasn't directly tied to the characters.

The problem was that the more I thought about the story, the more unfair to the world it seemed to just rip off Tolkien willy-nilly for my history. One idea lead to another and soon I was working on the ground floor trying to structure my own mythos.

Which presented their own problems.

This was kind of alien territory for me, because normally I only build a functional history for the characters I'm working with, and let the world history sit in the corner wallowing in it's own irrelevance. So suddenly I'm trying to determine the creation myths of this world. How different species came to be. The functional evolutionary chart of species and sub-speciation (which I came to realize is largely irrelevant in it's own right.... at least for the time being).

Most of all I had to figure out that mythical background to a core element of the story- The World Tree Ambers.

The tricky part of this is that in many mythos' things just exist. There's no grand story about the Golden Fleece before it had to be stolen. Mjolnir was basically just forged by a dwarf and given to Thor. Christianity has relics, but their histories are so muddy, and largely uninteresting, that they can't be looked to. So, there's no solid foundation for figuring out how these things came to be and why the do what they do.

So, I'm working with an item (3 items really) that was arbitrarily created in the early stages of this story. That item has now been ingrained into the story and events have been built around it. And I, functionally, have no clue exactly what this item actually does, why it does it, or where it's been for 800 years.

Now, I've managed to figure a couple of these things out. I know where it comes from (according to the myths of this world, and only one character actually knows the truth... so I have some time to determine whether the myth and the reality are anything alike), I know things it can do, I know what the bad guys want it for, and I know what the good guys want it for, I also know what the contagonist to the entire situation wants it for.

I still have no idea about the 800 year thing. None at all.

There's 3 ambers. The Dark Lord has had one of them for 750 years. He lost it 50 years before the story. Where have the other two been? I have no idea. How did they manage not to infect someone? Again I don't know.

I'm hoping that since those two ambers are not featured in the first book that I can figure out where they've been by the time they start showing up in later books.

Beside the ambers, I've had to figure out things like who the wizards are. How a tangential idea called the "Wall of Fate" came into existence, and how it fits into the narrative in a way that makes sense (cause when I thought of it I thought it was too cool not to use). I've had to figure out exactly who the Dark Lord is, both currently and originally, and how he came to be The Dark Lord. Who controls this Dark Lord? How does this Dark Lord have an group of wraths working for him?

All of these things just seemed to swoop in out of nowhere and demand my attention as much as character building did.

It's all alien to me, and I can only hope I piece it together enough to sell that it all fits together in a seamless package.

Fingers crossed.