In a non-fiction history book, Millennium: A History of the Last Thousand Years, a (temporarily) recurring element is "The Tower of Darkness".
Chapter 3 is even titled that, and it's fairly clearly referring to the Islamic empire or caliphate and its rule of a significant portion of Eurasia.
When searching to find whether the author made that phrase up, I found it's from Edward FitzGerald's English translation of The Rubaiyat of Omar Khayyam:
A Muezzin from the Tower of Darkness cries
"Fools! Your reward is neither Here nor There!"
Is there any direct evidence that the author of the history book got this phrase from the Rubaiyat?