The Vortex of Madness and Other Planar Perils
The Vortex of Madness and Other Planar Perils is a set of five adventures which are linked and set in the outer planes. The adventures were designed to be run separately from one another or run together, and are for high level adventurers.
These adventures are structured so that encounters do not just involve monsters to fight, although statistics are provided for combat. The eponymous adventure "The Vortex of Madness" involves a powerful prisoner held in a magical prison on the plane of Limbo; this location provides a tie-in adventure to connect all of the other locations together.
The Vortex of Madness and Other Planar Perils was reviewed by the online version of Pyramid on July 14, 2000. Although the cover proclaims that the adventures are intended "For High Level Heroes!", the reviewer notes that "there are no recommended levels to be found within. My estimation is that you would want to be at least 9th and probably at most 14th level to adventure in these areas -- lower level adventurers will not have the spells and abilities to handle the challenges within. Adventurers of much higher levels would need to have the challenges boosted in power to give them a run for their money."