So you’ve written your epic multi-book fantasy saga. Congratulations! Time to come up with the ending!
Was the final book finished before the first few were successfully published? Then the pressures on, and good luck with that!
Do you have a plan for the final book? Have you constantly adapted the plan as you developed the story, so that the first and the last books are nicely interknit without retconning anything? No? That’s impossible? Oh dear.
Well, you have to end this mess somehow. All the other books resolved themselves within themselves. And the stakes have to be higher than they ever were before now, or the readers will feel like they were cheated.
Ah: the key will turn out to have been something that, even though it is essential to the very survival of civilization, no one has mentioned before now! It’s the key to everything, but if someone had mentioned it before then the characters would have tried to deal with it before, so it has to be something that was kinda-sorta a secret. But not too secret, or the characters won’t be able to get enough information to complete the final big task.
So, let’s get a MacGuffin in here! They need to destroy something called a horcrux (which was totally a thing even when there were completely different explanations). Or they need to be the first to get to a tree! Or they need to do_____.
Yeah, it’s not perfect. It makes the final book feel disjointed from the rest of the series. It raises the stakes, but it does so in a cheap and dirty way rather than in a slow, long-term built-up way. But hey, better than no ending at all, right?