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.
Strictly on its own and without any contextual reference, 魔都, can only mean "Devil / Evil City" and calling it a "Metropolis" makes no semantic difference.
Since the author 村松梢風, Muramatsu Shōfu, is Japanese, then the Japanese translation for 魔都 should carry the intended meaning; and as I found out, 魔都 is "Mazu" which means "Magic City"
But WiKi says, "In his 1924 novel Mato (“Demon City”, 1924), he portrayed the dichotomy of Shanghai – a modern, beautiful, civilized façade, hiding a darker side populated by all manner of criminals and vice"
So there you have it. Pick the one meaning that suits your idea of Shanghai, now and in 1924, assuming you were alive in Shanghai then.