I've just started digging into the Fafhrd and the Gray Mouser stories, so this episode was really well-timed for me. This is the fourth of the stories I've read, but to be honest I have yet to be wowed by any of them, although Ill-Met in Lankhmar was by far the best. I can see the potential of the character work in the two protagonists, though, so I'm certainly keen to try more.
I found Brandon's comparison of this story to an episode of Stargate SG-1 interesting. There's something to that, although I feel like the SG-1 team have a bit more agency in the average episode than either Fafhrd or the Gray Mouser get here. The comparison that was running through my mind while I was reading the story, and even more so after I finished it, was with Lovecraft's Dagon. I'm not sure how widely known that story was when Leiber wrote The Sunken Land (a quick search suggests it had been published and re-published a few times before 1942), but it definitely feels like he was riffing on the same idea, if not outright writing his own Sword and Sorcery version of the story. I think Leiber's attempt ultimately suffers from the same problems as Lovecraft's - nothing really happens, and we don't get a proper look at the weirdness from below the depths. You can get away with that in a cosmic horror story (although Lovecraft barely did in Dagon), but in S&S you really need more action and a confrontation with a god-like being or sorcerer, especially if it's been hinted that the land was once ruled by some kind of malevolent cabal of magicians. The suggestion of re-writing the story to have Mouser come in rescue Fafhrd would go a long way to overcoming that issue.
All that said, I enjoyed the story well enough for what it is. Looking forward to reading more of these and getting more S&S on the podcast!