Slumbering Tsar: The Hidden Citadel, Part 6: Caverns of the Barrier

From the publisher's website:

Finally the veil of secrecy is lifted and the player characters fully understand the threat that the Hidden Citadel represents and what has truly been slumbering in ruined Tsar. In a race against time, the PCs pit themselves against a 10,000-year-old legacy and the vow of mighty Orcus to make the world his own in this, the climactic conclusion of The Slumbering Tsar Saga. PCs should be 20th level and above by the time they have completed this adventure.