Long ago the gods Tyrion and Malerion were borne to the mortal realms as beings of light and shadow respectively. The energies were anathema to each other making it impossible to enter the other's domain save for a few minutes. In Hysh Tyrion and Teclis scoured for any aelven souls and then searched in the Realm of Shyish without any success. Meanwhile in Ulgu Malerion and Morathi did the same and after being unsuccessful, entered the underworlds of Shyish concealed from the gaze of Nagash by a pall of shadow. While failing to find any aelven souls after years of searching they found an order of raven-worshiping monks in the distant reaches of Bonehallow who had recently offered aid to Tyrion and Teclis. 
These monks willingly led the aelf gods to one another and revealed their secrets. After much debated Tyrion and Malerion found common ground and combined the knowledge they gleaned from the raven monks and the ancient peoples of Shyish, uncovering a deep buried secret. Forgotten by almost all the knowledge had drifted into Shyish, where all things meet their end. The secret was Uhl-Gysh, the Hidden Gloaming, a sub-realm that was neither of light nor of darkness but yet of both at the same time.
|One realm is enlightened, the other is bleak, but in between there is a place the daemons come to seek..|
- 1: Malign Sorcery - Geminids of Uhl-Gysh, pg. 31
- 2: Geminids of Uhl-Gysh warscroll
- 3: Age of Sigmar: Core Book (2nd Edition) - A Hidden World, pg. 60-61