The Serpent Citadel is an adventure for characters of 7th or 8th level, and is the final adventure in the four-part Serpent Amphora Cycle. From the publisher:

A ritual gone horribly awry leads to a pursuit through the warped Hornsaw, where the Serpent Mother herself was torn asunder. With their own might and the assistance of unlikely allies, the heroes must recover a lost treasure and somehow stop the machinations of Mormos children, lest they awaken the titan.