The City of Butterflies Gazetteer (2002)

From Introduction:

The City of Butterflies, also called Hu Die Cheng, is the capital of the Jade Emperor's province. It is dominated by a giant palace that is built on magically strengthened pillars, hovering 9 feet above the ground. Many rich nobles and bureaucrats try to emulate this style and due to the many buildings, an undercity of sorts has developed. Everything is constructed of various metals, jade, or silk and indeed the city almost looks like the wing of a butterfly from afar. However, the name Hu Die Cheng comes from the common form of communication in the city - tiny butterfly-shaped constructs. Since they are all provided by the Jade Emperor himself, he is always able to keep track of conspiracies against him.

Hu Die Cheng offers many adventuring opportunities since it is rife with intrigue. Also, every 12th full moon, the city disappears into the shadow plane for a whole night, as part of a curse that has befallen its ruler. Nobody knows the nature of this curse but it is obvious that the ruler is somehow involved as the shadow creatures are drawn to him and yet he never leaves the city at the time of the shift. During the period of extraplanar disruption every able hand is needed to defend the city and pay is good, even though one has to live with the korobokurus in the undercity. Since evil spirits often infiltrate the real world during this time or possess people, there is always something to do here.