There is a subrealm of the Dreamlands known as the Nightmare dimension closely associated with Mystara. James Mishler has an article about the plane right here.
Time has very little meaning in certain locations of the transitive planes for they are closely allied with the psychic powers & abilities of the mind. Here the connections between Mystera, Zothique, & Hyperborea become far more obvious. This all ties into the dark occult history of the Cynidicea & its ties with The Dungeon Master's Guide to Cynidicea article on the Vaults of Pandius. This helps to explain the presence of Zargon on the surface of Hyperborea & its cults deep connections to the Diamond Desert Underborea area.
Zargon's cults are insidious, dark, & very dangerous. They've crossed planes & their wizards should not be under estimated. Zargon is one of the entities that's survived the events of Ragnarok on my version of Hyperborea. The old gods are dead & there a few survivors which leaves room for the entrance of Lovecraftian abominations like Zargon & his ilk.
Thor and the Midgard Serpent (by Emil Doepler, 1905)
Zargon is building his base of power on Hyperborea by the time of the events of X5. The PC's are going to have reached a solid level in the campaign path that I've laid out. There are a wide variety of factions vying to take out the political infrastructure of the fragile governments of Hyperborea. The plane of Nightmares has all of the qualities of chaos that would be perfectly in tune to cause chaos & strife where & when as needed.
Hule is the perfect an oppressive theocracy of doom to drag down the entire works of Hyperborea leaving the entire world open for take over by the forces of Zargon & its cults. I've never run the Cynidiceans as a joke at all but instead like a horror movie cult akin to something we'd see in a Call of Cthulhu adventure. They've had their sanity shattered as a people & the remains are holding on to maybe the last bits of sanity. The Cynidiceans looked long & hard into the "Dimensional Vortex," which is defined as "the void between all dimensions."
Something deep & sinister answered them back & its been feeding on them ever since. Zargon is only the latest in a long line of horrors that has crossed into the realm of Hyperborea. One or more of the PC's can gain lycanthropy over the course of X5. I strongly suggest that the party allow it to give the players a fighting chance.
The key to "X5 Temple of Death" By David 'Zeb' Cook. is realizing that there is far more bubbling below the surface. I had details of the Forgotten Fane of the Coiled Goddess show up in X5 such as the legend of the treasure called the Feathered Crown of Nanasa. This led to all kinds of interesting twists & turns as the players wanted to abandoned the quest of X5 to find this artifact.
This leads deeply into the fact that serpent men are behind some of the major threats of the Forgotten Fane. But their connection to both the Dreamlands & the legacy of X5 is thousands of years to late. There are much more dangerous occult & ritual connections between these modules. We'll get into those next time.