I mentioned, a post or two ago, that Fritz Leiber's 'Adept's Gambit' is one of the stranger stories about Fafhrd and the Gray Mouser. What makes it so strange? It takes place on Earth.
All of the other tales of the Twain take place on the invented other-dimensional world of Newhon (That's No When spelled backwards). I can remember being somewhat confused when I first read the story back in the early 1970s. At that time Ace books was publishing the Fafhrd and Mouser stories in five volumes which gathered the original magazine stories and included new material written especially for the collections. All of the books had the word 'swords' in the title. Swords Against Death, Swords and Deviltry, etc etc. Adept's Gambit appears in the third volume, Swords in the Mist. (Later two more volumes would appear.)
Now being a good little reader, I was reading the books in the established chronological order (Once I had learned of it. I read Swords Against Wizardry first.), so I'd already read three entire volumes of the Twain's adventures on Newhon before I got to Gambit. Needless to say it was a bit odd to suddenly find Fafhrd and the Mouser in ancient Tyre, running into Egyptian priests and traveling to Alexandria. A new story called 'The Wrong Branch' had been tacked on to the beginning of Gambit. It explained that the boys had wandered into the wrong corridor of the labyrinthine caverns of the sorcerer Ningauble of the Seven Eyes and had accidentally crossed from their own universe into ours. The crossover readjusted their memories so that they thought they had always lived on Earth instead of Newhon and did most of their adventuring in Tyre instead of the city of Lankhmar. Very handy, though it seemed to me at the time to be a lot of trouble to go to for one adventure on Earth. (technically the boys visited Earth again in an issue of the Wonder Woman comic book of all places, but that's a story for another time.)
Years later I learned the real reason for the sojourn to Earth. Adept's Gambit was actually the first Fafhrd and the Gray Mouser story that Fritz Leiber ever wrote, though not the first published. When Leiber was originally conceiving the characters along with his friend Harry Otto Fischer, he wasn't exactly sure of their origins and so he set the story in recorded history. Thus the Twain were originally intended to be from our world, albeit an Earth where sorcery really worked in the past. Lieber submitted the story to Weird Tales, but it was rejected by Farnsworth Wright. This put Leiber in good company because Wright had at times rejected the work of Robert E. Howard, H.P. Lovecraft, and Clark Ashton Smith.
The story remained unpublished until it appeared in 'Night's Black Agents', the 1947 Arkham House collection of Leiber's work. By that time, Lieber had published several Fafhrd and Mouser stories in the magazine Unknown and their fictional lives on Newhon were already well established, making Adept's Gambit a bit hard to fit into continuity. When Ace published Swords in the Mist in 1968, Lieber took the opportunity to write 'The Wrong Branch' thereby making Adept's Gambit an official part of the continuity, explaining away its setting in the Seleucid Empire (323-281 BC) with a nice bit of sorcerous slight of hand.
Another weird element to Gambit is the constant mention of the Elder Gods. Leiber was corresponding with H.P. Lovecraft during the writing of Gambit and in fact, Lovecraft read and critiqued the manuscript and showed it around to other members of the 'Lovecraft circle'. Reportedly the manuscript originally contained even more references to Lovecraft's Cthulhu mythos, but those were removed before the story was published. But the references to the Elder Gods remain. See? I told you it was a strange story.