Thousands of years ago, the sages of Penglai found a way to open gates from one world to another. This new world was much like their own, but different in small ways. One of the ways was that this new world's Emperor was faster to act, and conquered Penglai in a swift act of betrayal - he assassinated the emperor of Penglai and took his place, then 'conquered' his home nation. Stories sometimes vary about the details, including which emperor joined the worlds together, but this was the founding of the Jade Empire.
From the new world, in time, a gate was opened to a third world, even more different from Penglai, but the combined forces of the two worlds were more than enough to overwhelm the third, and this became the pattern. As each new gate was discovered (usually a process of years, sometimes decades) the new world was offered a choice to join willingly, or be crushed under the boot of the Empire.
Things have not always gone smoothly. The simple act of administrating an empire of this size (now almost 100 worlds, and over 1000 nations) is a nightmare. The magistrates and the bureaucracy keep things moving well enough, but the further one gets from the imperial capitol, the more autonomy the magistrates have, for better or for worse.
Jade City was one far-flung arm of the Hundred Worlds And Thousand Nations Of The Eternal Empire of Penglai, of no particular note until Amber made contact with it and began trading. Most in Amber are not truly aware of the extent of the empire, as the use of the provincial name indicates
While the empire can nominally field armies of truly unbelievable size, they have few magics for making the logistics or transportation any easier, so it is difficult to bring the full weight of the empire to bear. But difficult is not impossible - the full extent of Jade's potential has never truly been tested.
The greatest challenge to Jade comes from within. While many of the worlds they've discovered have been brought in peacefully, that has not always been so, especially in the case of world populated by creatures and beings anthetical to the empire. When faced with an unbeatable foe, the empire has sealed gates and left worlds to their own fate, but this is only done when there is no other way. Usually the empire fights and wins, but the remnants of these wars haunt the shadows of the empire.
The Empire is sufficiently large that it is hard to say it has an opinion of Amber (to say nothing of the Golden Circle). Most of the Empire has never even heard of it, and until it came into contact with Amber, the City of Jade was of only moderate importance as a regional capital. Jade City has grown in leaps and bounds, and this is creating tensions within the empire. The prospect of an external ally (or rival) is a strange idea, and its meaning is still being worked out. Of course, this has been the status of things for centuries, but change comes slowly in the empire.
Because of the limitations of formal government, informal government abounds in the form of secret societies. Jade is so riddled with them that the question is not whether someone is a member, but rather which societies he is a member of?
Jade is a robust trading partner, with broad supplies and broad needs. What it lacks in specialty, it makes up in volume. Additionally, Jade has steadily become the dominant force in portside smuggling in Amber.
Gate magic is interesting, but it's subject to so many limitations as to be mostly color. A place for a gate must be found, a process that can take years if there even is anything to be found, and then it must be painstakingly constructed wherever found, which may be a long way from civilization (to say nothing of another gate). Once it's built, there's no controlling where it opens, though in terms of shadow-travel, it will not be "far" so existing gates aren't even much use for fast travel.
That said, there are said to be a number of mystical practices within the empire, some no more than local superstition, some holding secrets that span the empire.
In Yoru was a gate only recently re-opened after being left to the devices of the demons and monsters just outside that place for generations. It's stable now, but that stability is tenuous. There's a large military contingent on hand near the gate, and the fact that that also puts them near the gate to Jade is not something that has passed unnoticed.
Penglai is a romp through the Asia of myth and history. Demons linger from old wars, and various iconic images of pan-asian myth can be found throughout the many worlds, with a unifying theme akin to a mythic imperial china. Going to some out of the way world to face an impossible challenge or court a local god is business as usual for a hero in Penglai.
Penglai is also on the cusp of change. At some point, Penglai will either experience a schism or test its might against Amber. This conflict is played out in a thousand small ways among the various secret societies, and a kung fu and intrigue laden plot that benefits one secret society or another can have great long term impact. Players are encouraged to make up secret societies with ludicrous names at the drop of a hat. At some point soon, the world of Jade may attempt to break free of Amber.
Wu-Fei is the propco for Jade and Penglai, and there is also a wiki at http://jade.wikidot.com.