Shangdu was the capital of the old Mongol Empire. At its zenith in the 13th Century, when Kublai Kahn was emperor, it was home to at least 100,000 people with maybe a million more traders, civil servants, and diplomats arriving and leaving over the course of the year.
In the late 14th century, Shangdu was sacked and torched by the Ming army and then abandoned. For centuries, only a few broken temple ruins could be seen. The city lived on, though, in folklore and legend as 'Xanadu'.
Xanadu is now a UNESCO protected site in China. Presently, as in Kublai Kahn’s time, the area is mostly grassland but year by year the rains become rarer. In the future the site may be but deserted arid wasteland.
Every March, Yellow Dragons wreak havoc on the Xanadu site. These are violent sandstorms whipped up from the Gobi desert. They often block out the sun for weeks at a time.
Over the decades they’ve become worse and worse as the Gobi gets closer and closer to the Xanadu site,and evermore soil is denuded of grass and then desiccated. In the not-too-distant future, the Gobi Desert will surely have invaded completely. Welcome to Franken-Shangdu.