The Oathbreaker

From the introduction: This adventure can be can be easily customized to a party of any size or level, or run as a level 8 adventure for 5 players. It can work as a small side adventure in an existing campaign, as a standalone one-shot, or the start of a new adventure. Use the links to the tools in their relevant section to customize the adventure to your party. ... The Blackguard Amduscia has made a deal with demons to conquer the lands and install himself as the new king. He has made his lair in a tropical oasis of hot springs, where the fabric between the planes has become unstable. From this base he has been seeking a powerful artifact weapon to bolster his army and destroy the kingdom. Recently a royal researcher has gone to the springs to study them, but hasn't returned.