Wall of Darkness
The Wall of Darkness is a powerful essence barrier separating Shadow World, rendering the eastern hemisphere almost impossible to reach. It was formed as a consequence of the Eyes of Utha, which are powerful artifacts designed to suppress the interdimensional rift following the First Era cataclysm. The Wall of Darkness is actually an interphase between this and the "negative" universe of the Unlife, and the barrier extends through the core of the planet. The Lords of Essaence were masters of gate magic, discovering many other realities they felt paled to their own. In the war to overthrow their wicked Empress Kadaena, there were vast hordes of demonic monsters, which came from other planes of existence or possibly created artificially for the purpose of forming avatars for everlasting existence. These portals were called the Gates of the Void. The war ended when Kadaena was decapitated and by some accounts was sucked into the Void. Many of her servants were imprisoned on the other side of it.
The instability of excessive gate magic was worsened as the forces of Utha, cousin of Kadaena and father of Andraax, unleashed powerful channels of Essaence to wrack the world in destroying her servants. The fabric of reality was badly warped by the titanic scale of power unleashed in the war, with many violent and unhealed tears which continued to allow access by extra-planar horrors. The Eyes were constructed to suppress the formation of random portals, as well as shield the world from the powerful flow storms of space which would tear them open. They keep the world on the edge of the rift, which is the source of magic, but held it from falling into the chaotic realms of pure essence. The failure of the Eyes of Utha could cause the world to be destroyed or make the world magically inert.
The Wall of Darkness was formed as a result of sealing the wound in reality. There were servants of Kadaena who survived but were trapped on the other side of the barrier, as well as the hideous otherworldly beings let loose from the portals which still existed beyond the wall. Most people assume the eastern hemisphere is a place of horrible evil, possibly some negative universe of Unlife and "good" Essaence is limited to small pockets, even though no one is able to travel there to witness it. (There are only a few exceptions, such as the most powerful dragon lord, and the avatars of the moon gods. Andraax went mad and traveled there at the end of the Wars of Dominion. Those who would attempt it would likely be transported to somewhere on the western hemisphere, or possibly one of the other planes entirely. The Wall of Darkness is thus not merely an essence barrier.) The Lords of Orhan (Liabo) spent thousands of years closing these portals and slaughtering the demonic in order to repopulate the world.
Behind the Scenes
These correspond to the Eye of the Drake and The Rift in the modern history. The Elemental Confluence is essentially the same concept as the interdimensional rift of Shadow World, with Alusius stating it was the ultimate source of elemental magic in this universe. (Similarly, the magic of Elanthia is supposed to run through Mount Aenatumgana, related to the old Koar theology as Master of Mana.) In theory it would have been the kind of thing the Eye was designed to deflect. The Eye of the Drake was destroyed in 5098 Modern Era, while the northern Eye of Utha is under siege in the present of the Third Era.
The Wall of Darkness itself has no analog in Elanthia. The world is known to be round and navigable by ship, with the Shattered Continent reached by sailing both east and west from Elanith. However, the town guard Quin Telaren speaks of a "curtain of light", which was incorporated much later as a lunar conjunction phenomenon with the Red Forest. This might have been an allusion to "A Game of Thrones", the first book of A Song of Ice and Fire which was published in 1996 and became fantasy book of the year, where Bran has his first dream vision with the three-eyed crow while in a coma after being pushed out a tower window. He is falling endlessly, witnessing dead dreamers, and recovering his childhood memories like the death mechanics of purgatory. Bran witnesses a "curtain of light" at the northern edge of the world, beyond which is the "heart of winter". The curtain of light would be the montage of colors in purgatory, or Drake's Shrine rainbows, but could also be interpreted as a Wall of Darkness analog.