Isles of transfer
Isles of Transfer were a mysterious phenomenon in the Shadow World history that defied explanation even by the Loremasters. There were islands which existed in multiple places at the same time, or even multiple times in the same place and several times and places simultaneously. These were mostly found in the Malvin Tesea (Iruaric: Endless Sea) on the edge of the hemisphere, perhaps related somehow to the Wall of Darkness. The isles will often drift through space and time, but may nevertheless be consistently anchored to the same set of universes.
These properties made them extremely dangerous, and they were widely avoided by travelers who knew better. The Navigators made sure to stay far away from them because of their instability, which is hinted at by the guide of Selias Jodame in the Shadow Valley story. They were much like random portals, except on a far more lasting, larger, and more threatening scale.
Behind the Scenes
The isle of transfer phenomenon is strikingly similar to the Shadow Valley story, which should have been located not far from the krolvin warfarer settlement. It was instead anchored deep underground a few hundred miles away, adjacent to the shrine of Bandur Etrevion in The Graveyard. It was haunted by a powerful serpent demon that was notorious for bursting into other planes of existence at will, with infestation originally by dust beetles of extra-planar origin. It is likely related to the dreamlands premise below The Graveyard and the moon nature of The Broken Lands in spite of being another reality. In the mineshaft the lighting method is glowing moss just like in those other places, which is Lovecraftian, and the miners died from being phased into the rock walls which had apparently shifted in space.
It is worth noting that the Luor'ka'tai (Iruaric: "Pillar of the Gods") was a primordial warper of spacetime, located in Thuul near the Wall of Darkness. (Those specific words were conspicuously missing in the Iruaric glossary produced for The Broken Lands, but that might be coincidental because of minor differences in the glossary between editions of the source books.) The pillar was formed by a tiny black hole in the cataclysm where the Lords of Orhan arrived, causing the interdimensional rift that originated magic. The Broken Lands is made up of symbolic parallels to locations in Lovecraft mythos stories, such as the Plateau of Leng and Vaults of Zin, that all happen to coexist in multiple realities simultaneously. It is just a question of what extent that parallel was intentional in terms of the fine details of the mythos.
In the modern history there was a rare lunar alignment of Liabo and Lornon that causes a curtain of light where the Red Forest of the Ilvari comes back into conjunction with our own time and space where it belongs. However, there was a complication when this celestial alignment happened in 5116 Modern Era, resulting from the interplanar distortions of the Elemental Confluence. This caused the Red Forest to coexist with our plane permanently again, with manifestations of the same forest on opposite ends of the continent. They are fog filled and demonic, poisonous, serpent, and dream themed.