From the February 1943 of UNKNOWN, this was the fifth and final Fafhrd and Gray Mouser story to appear in that magazine. There is nothing else quite like these stories and I would hate to see anyone attempt to continue them after Fritz Leiber's death. Just speaking personally, I thought the episodes he turned out after 1968 or so lacked some of the substance of the earlier tales. They become more indulgent, more concerned with pretty wordplay and baroque images than actual storytelling.
But the early stories...! Each is a little gem that has to be read carefully to catch all the tiny puns and wry asides. These were Leiber's tales of two unlikely heroes in the strange world of Newhon (spelled backwards, it's not quite Nowhen) and its ancient city of Lankhmar. Fafhrd is a big hulking barbarian from the Cold Wastes, a straightforward mercenary sort of warrior with a huge broadsword.
His partner in crime is harder to categorize and possibly unique in heroic fantasy. Small and wily, given to stealth and cunning rather than brawling and bludgeoning, the Gray Mouser has an insidious charm. Both heroes are more complex than the standard sword and sorcery type. They have a full range of human emotion, actually fall in and out of love as real folk do, have wide interests and skills, and tend to get by on luck and quick thinking rather than brute force.
"Thieves' House" starts off with a conventional premise. A member of the Thieves' Guild has made a deal with Fafhrd and the Mouser to steal the jeweled skull and bony hands of a long-dead member of the Guild. (The theft requires deftness and skill, and there is this guardian beast to slay, so the help of the two heroes is unfortunately necessary.) Professional thieves being no better than they have to be, he immediately double-crosses them (before they can do the same to him) and flees to the refuge of Thieves' House. Unhappy with all this, Fafhrd and the Mouser give chase.
Here is where the story takes several wild twists. Hitting his head a few times more than seems necessary to the plot, Fafhrd stumbles through the windy black passageways and stairwells deep into the earth. Thieves' House is ancient and vast, and its present tenants do not know all its secrets. There in the airless, dusty darkness are, well, things waiting.
Meanwhile, deciding to rescue his partner, the Gray Mouser is forced to try disguising himself as an old woman fortuneteller (Conan never resorted to this) and gets caught up in his act. He finds the agebrowned skull and has the bright idea of faking a supernatural occurence with it to panic the thieves. But, in Lankhmar, it's not wise to imitate the Unknown.....
Leiber's skill with language is priceless. Just when you think you've seen every possible macabre image, you come across Fafhrd in the deep catacombs snatching at a clatter in the air and finding there are bat skeletons flying around him in the blackness. (Eeesh...) The two heroes have actual personalities, not just the blind fury or lust of most pulp barbarians, and they are believable as old pals who know each other's shortcomings but are used to them. I like the way Fafhrd keeps making halfhearted attempts at rationally explaining all the weird goings on and the Mouser's devious way of looking at things.
Although they don't appear in this particular story, two of my favorite wizards in all fiction are the patrons of Fafhrd and the Mouser: Ningauble of the Seven Eyes and Sheelba of the Eyeless Face. Leiber is always stirred to his best efforts when dealing with these two, and the little hints we get as to their real natures and appearances are tantalizing.