The Gateway Seals

The story says that the end of the Demon Age, 10,000 years ago came with a portion of the crystalline Dome of the Heavens falling to the ground and creating a great cataclysm and tumult. The Inner Sea was created, the rings of mountains rose up, while the mountains of old were cast down, great volcanoes and fires were everywhere, while other parts of the land sunk and were drowned or were buried in avalanche and the changing of the land.
It is said that reality and the world fractured. Parts became pocket realms - not part of anywhere else. Demons and otherworldly creatures - like the Yokai were trapped in these places. In the long ages since, they have found cracks. Small fissures that one or a few might crawl through. Or Warp Storms reach into their domains and pull parts to the world. They have build armies and seek ways to come back.
Before the Empire, even in the Pagan days there were larger holes. Temples and Shrines were made to keep watch on them and great magics and arts were used to make Seals that closed the holes. To this day the Temples and Shrines still guard the Seals, though which and where are either lost or kept as the utmost secrets.
Somewhere on the Holy island of Kuan ( Kuan ), the very Core of the Empire, there is rumored to be one. Once kept by the Gods, they were delivered to the Emperor and the empire to watch and guard. The Demons, Yokai, and the other dwellers in the fractures want out and seek these Seals, or information on them.
Mad Monk Lazra of Uxrighia wrote in "The Book of Ancients" in 549 ( Empire HIstoric Timeline ) that he had seen one of these Gate Seals:

Tall as two persons. Set in a seal of salt. Carved all of jade and set with runes of the five magical metals in five circles. Also pierced by rings of the five metals in the jade and linked to Dragon bone. From the rings ran iron chains to anchors in the five points of a pentagram inscribed in the wall.

The Temple at Uxrighia was attacked and destroyed by creatures in 627. No Gate is known there. Lazra never said where he had seen the Seal.

Spread

It is said, though not with certainty, that the Deadlords (Dead Kings/ Titans ) of Damamilia, the Lizardkings ( Octnon ) and the Centaurs also have Gate Seals in their keeping in the Taurien States . The Harrower is rumored to control a Gate to the Underworld in the The Harrowland and year by year performs rituals and blood sacrifices to rip it wider. Only Blood mages, Necromancers, the Vampire minions and other evil servants of the Harrower may know for sure.
It is known that small fissures may be found all over the region of the Inner Sea.

Variations & Mutation

Some Gates are said to be guarded by powerful spirits and other creatures, such as Dragons. It is hard to know because the lore is myth and legends and truth is kept as a close secret by mysterious orders within the ranks of Priests and holy warriors. Or are kept by strange creatures such as the DeadLords or the LizardKings....or things stranger still, in the wilds and ancient ruins from the shattered demon age. Even the Centaurs and Minotaur will not reveal where these things lay or if they even know of them.
The lore is not stories of the common folk. Mostly. The Gate of Kuan is known to have been given by the Gods to humans, specifically the Emperors, to guard. For this there are stories, legends and supposed witnesses. The story of the first Imperial line, the Torga, ends with an assault on the Gate and prevented by the Torga heir's sacrifice.
Most of the lore is passing references and strange puzzles in the works of sages and lore masters. The Fae claim some knowledge of Gates and Seals in parts of the world. Those who claim to possess the lore say it is, and should be kept, from human knowledge.

In Literature

An epic on the fall of the Torga emperors is a popular story and was set to a replicated performance in tribute to their noble duty and sacrifice by scribes in 783.  The theater work by playwrite Kiji Va-shir in 909 is considered the best and classic version of the story.

In Art

Owoda Tzizo's painting of the battle of the Gate of Kuan was produced in 817 and is considered the inspiration of Kiji Va-Shir's final battle scene where the Torga line battles the demons to the last while the Gods watch and judge them worthy by their sacrificing themselves to protect the Gate and the Xian line takes up guardianship from the dying Torga Heir.