Mark
CreativeMountainGames.com
Grab the free Cooperative Dungeon Collection which includes a Cooperative Dungeon 01 - Terror and Blasphemy, a 14 to 15 level adventure, that could be adjusted to fit.
Three possible ones from War of the Burning Sky:
Tears of the Burning Sky for levels 11-12. The heroes head into a necromantic firestorm, to find an artifact lost when a magical siege went awry. The artifact lies in a castle whose attackers and defenders were transformed into burning undead, and now the heroes have to race against two other groups who want the artifact -- one group of gnomish treasure hunters, the other of soldiers who escaped the first conflagration.
As part of the WotBS campaign saga, the artifact has a specific identity, but you could easily change it to something else for your campaign. If you want a huge dungeon full of lots of undead, but with fun set-piece encounters so it's not just a grind, you should check this one out.
The Trial of Echoed Souls for levels 13-14. The heroes need to track down a trio of assassins who absconded with the body of an important person whom the PCs want to bring back to life. The trail leads them to a haunted elvish forest, where militant elves are trying to find the same assassins because they too want to bring him back to life, albeit for more sinister reasons. The ghosts of the forest's elves, slaughtered by genocide, complicate matters, and the whole search climaxes with a Predator-esque chase of a shadowdancer through a trap-riddled tree city.
In the end, though, they discover the assassins actually consumed the soul of the man they killed, and so the heroes head to a psionic temple where it is rumored it is possible to reshape or recreate a soul. This adventure I co-wrote with Ari Marmell, and I think it turned out awesome.
O Wintry Song of Agony for level 15. Have you ever wanted to let your PCs do their own version of the "destroy the Death Star" mission in Return of the Jedi? In this adventure, the heroes are the strike force at the head of an army trying to take out an installation that can aim magic anywhere in the world. Hidden in a frozen valley, and built over a gulag full of insane wizards whose suffering provides its power source, the enemy fortress has defenses that only high-level heroes can hope to face and survive.
All the adventures are available in PDF or print.
To run any WotBS adventure stand-alone, it helps to have a nation whose military can be the enemy. You don't necessarily have to run them as part of a war plot-line, but if you don't want a big group of enemies to come out of nowhere, you'll want to drop hints ahead of time that group X is bad, and might be up to no good.
We made sure all of the adventures can stand alone. Each of the three I mentioned feature some NPCs and plot elements from previous adventures, to provide continuity and allow for more roleplaying opportunities, but you could easily switch those NPCs for ones from your game, or just have them be strangers.
The 8th adventure would require the most revision, but most of that would be in backstory, because the adventure serves as a major 'revelation point' in the campaign, when you rescue someone important from the prison.
The 6th adventure would be the most stand-alone of the three, because it's basically a dungeon crawl looking for whatever item you want to slot in instead of the Torch of the Burning Sky.
The 7th adventure I think is the most original of the bunch, but the final act of the adventure (the actual 'Trial of Echoed Souls') involves fighting in a psionic dreamscape manifested from the memories of the heroes and their enemies. Since your players probably won't have fled a city under siege, gone through a burning forest, ended a magical hurricane, and so on, you'd have to revise those elements.
Send me an email at ryanznock@gmail.com, and I'll send you one of the pdfs; your choice.