At this point, you've been running the game for awhile, somewhere between 7 and 10 sessions. It's time to gear up for the climax. Yeah, I'm assuming this campaign will last until session 20. That's a Herculean effort, be proud of that!
If you're a beginner DM, I wouldn't try to go over... not until you've been doing this for years. And even then, I feel like a 20-session campaign is a good place to stop. One day, I'm going to run a campaign that continues for years, but even I'm not ready for that - too many responsibilities!
I. Bring Together
A. Story Forge - As the GM, you didn't craft a story (plot) before the adventuring took place. Now that the PCs are mid-level, the half-told story can start to be understood.
B. Tying disparate threads together - The thing your 3rd henchman said about the azure spider might have something to do with the blue webbed caves of Aza'akeen. Find patterns and sew seemingly random elements together.
C. PC backstory - Up until now, I hope you've mostly ignored whatever history the player-characters have been yammering about. But this is when it starts to matter - use something from their past to either complicate the present or make it more dramatic.
II. Godlike Power
A. Special Abilities - As the PCs have reached mid-level, it's time they started REALLY kicking ass! Don't feel you need to go all-out "new school" with feats, prestige classes, and class-based talents up the wa'azoo. Nevertheless, as the GM, you should dole out something kind of awesome to reward players and their characters for sticking it out this long.
B. Artifacts & Relics - Somewhere in the realm, or outside of it, there's got to be an unbelievable magic item, piece of high-tech, or super-science contraption. The PCs can quest for it before they invade the Big Bad in his lair.
C. Evil has its own ultimate weapons - The monsters and deadly NPCs pitted against the adventurers also have godlike powers. Unleash them upon the PCs, or at least threaten to rip into them.
III. New Is Always Better
A. Non-standard - If you've been relying on stuff you've read, watched, or stolen from other campaign settings, that shit ends here. It's time to create your own content - new monsters, new traps, new magic items, new treasure, new places, new NPCs, etc.
B. Shocking reveal - There's something that's always been nagging you from the very beginning about this campaign setting. Remember the waterfall mist and time/space doorway from Land of the Lost? That's what I'm talking about. Time to turn things on their head - what the PCs accepted as true suddenly proves to be an utter lie! Revel in your revelations!!!
C. Big Bad in his lair - The final confrontation is just around the corner. Gearing up for that confrontation will take a couple sessions and then actually storming The Beast's black fortress (or whatever) will probably also take multiple sessions.
Hope you enjoyed all 3 parts, hoss. Thanks for reading, commenting, and sharing!