The Lost City of Idras

From the introduction: The Lost City of Idras is intended for four characters with an average party level (APL) of 12. Characters who complete this adventure should earn enough experience to reach halfway to 13th level. Deep beneath the waves lies a city of horrific mer-creatures protecting a dark treasure. This campaign takes place in Arden, although it can be set in your own world. Any party composition should be able to finish the adventure with intelligent play.