Sa'kain is an important unnatural comet of extra-planar origins in the Shadow World history. When the Lords of Orhan (Liabo) first arrived in this universe from the cataclysm of a higher reality in -80,000 First Era, the rift between these universes threatened to collapse both dimensions and they were forced to seal themselves on this side of it. They chose the largest moon as the physical anchor for their primarily non-corporeal form of existence, making it formidable and unapproachable, and cloaked it further in illusions and misdirection to keep away the Lords of Essaence. The interdimensional violence included a tiny black hole passing through Kulthea causing a tall spire of black crystal and metal to rip miles into the sky, located in central Thuul near where the Wall of Darkness would be later. This Luor'ka'tai (Iruaric: "Pillar of the Gods") is surrounded with powerful essence disturbances warping spacetime. This is potentially relevant to The Broken Lands and the isles of transfer phenomenon.
The comet first appeared in -16,000 First Era and disrupted the seal on the interdimensional rift, making the world vulnerable to extra-planar intrusions from the Void. The Lords of Orhan were unable to seal this interface, so the world was irradiated in the Essaence of the other universe. This would become the basis of magic. The Lords of Essaence weakened the barriers between worlds with portals. It was this vulnerability that first allowed "dark essence" and the demonic into the world, which would eventually corrupt the imperial line until the breaking point in the reign of the Empress Kadaena.
When the world was nearly destroyed by the Empress Kadaena and her cousin Utha in the civil war of the Lords of Essaence, the fabric of reality was badly damaged from the scale of powers that were unleashed. Sealing the "Gates of the Void" to some large extent required building the Eyes of Utha, powerful gemstones at the poles, which shielded the planet from the flow storms of space and formed the Wall of Darkness. Unfortunately, the comet Sa'kain would periodically interfere with and ultimately damage the Eyes, compounding the hazard every fifteen hundred years. Sa'kain weakens the barriers between universes with each visit. If the Eyes of Utha fail it will either plunge the world into a chaotic demonic reality, where flow storms are constant, or magic itself will fail entirely in this universe.
In 450 Second Era this resulted in the first arrival of the Dark Gods from more chaotic demonic existences, or possibly an ancient interdimensional prison, ripping open dormant Lord of Essaence portals on the moon Charon (Lornon). The Lords of Orhan form their first avatars and reveal themselves to the world as a counterweight against these new powers. The comet makes the worldly access of the Dark Lords that much easier in 1950 Second Era, though they hide it for several thousand years to prepare for war, and there is the first appearance of servants of the Unlife itself.
When the comet returned in 6450 Second Era it ripped open gateways to the demonic throughout the world, and the Dark Lords unleashed the dark host they had been building for thousands of years. Other conquerors such as Lorgalis reluctantly side with them, spawning the several hundred year Wars of Dominion. The Lords of Orhan end up intervening at the end, banishing the dark gods back to their moon and sealing their gateways. In the Third Era the Eye of the Utha at the north pole was failing, leading to a power struggle involving factions seeking to destroy the world.