If you and your players are experienced enough to handle high level play (there's a lot to track at that level), then I might suggest picking up one or both of Dragons Unleashed or Undead Unleashed. You'll find very affordable sourcebooks that contain multiple entries for high level dragons and their lairs, or high level undead. Each lair contains history and background that can be extrapolated into a several or many sessions-long game.
The Pathfinder Modules page on PathfinderWiki is really useful for finding officially published modules that best suit your needs. That said, the following modules are in the range you're looking for.
Blood of Dragonscar is for 15th level characters. I believe this module uses 3.5 edition rules, so may require some conversion.
The Moonscar is for 16th level characters.
The Witchwar Legacy is for 17th level characters.
If you read the thread devoted to the Witchwar, you'll find that one high level caster can basically solo the whole module. It's extremely swingy that way.
The Moonscar is probably more fun, because it's more sandboxy and you've great excuses to throw in more challenging stuff for your players.
==Aelryinth
The Demon Within was written for D&D 3.5 and is only 11th level (high for me, but perhaps not for you).
In my admittedly limited experience, this is one of the best stories within a high level module I've ever run. There was plot/secrets to uncover, an excellent gradual building of threat and a great time-sensitive, "It's up to us to sort this out" mood throughout.
It also appears to be currently (still?) on sale for $2+postage, so how can you go wrong?
Coliseum Morpheuon is amazing.
Rappan Athuk is a 3rd party dungeon with plenty of high level play. You can just run it as-is, have them plow through the easier areas and make a name for themselves, maybe.