I know it is referring to Shanghai, and Baidu has a handy explanation of its origins here, but how would you translate it? "The Mystic Capital"? "The Devilish Capital"? In the theme of 帝都 to describe Beijing as the "The Imperial capital" etc.
If we disregard the existing historic translation
Demon City, and want to come up with a more, perhaps "modern" English translation to convey what early 20th century Shanghai was like, then what about:
Sin City. It is still used to describe historic Shanghai, as seen in these examples:
Sin City Shanghai | For a short time in the beginning of the 20th century this city ... Shanghai was THE island of wickedness on the Chinese continent.
Sin cities in which you can pursue vice and debauchery have existed throughout history, from Rome’s ..., to Shanghai where you could float for days in opium dens high on sex and drugs.
Why not "Mordor"... I have always believed this is the only correct transl(iter)ation.