THE CIRCLE CURSE
If this is your first Fafhrd and the Gray Mouser, The Circle Curse works as an excellent introduction to the two adventurers (and two wizards that will return in later stories.) It sets the main theme and a bit of the mood that will guide the rest of the stories, sketching the reasons for their sudden departure from the city of Lankhmar. There is probably no more classical motive for a wandering hero than a broken heart, and in this case, this driving passion also humanizes the characters and gives them a more complex personality, something important once they start their less unsavory exploits.
This story negates the traditional direction taken for granted in most fantasy, where the narrative begins in a small physical location, almost a prison for the main protagonist who yearns to discover what lies beyond the literal or metaphorical walls of his homeland. On the other hand, Fafhrd and the Gray Mouser aren't really yearning for adventures as much as they are fleeing from their own demons. But that world beyond Lankhmar and their travels, something that could have been the story for a whole book, are barely described in a few short but evocative paragraphs. The journey doesn't bring them the peace they need, and since they have traveled all around the world and the pain has not yet subsided, the solution seems inevitable: they must return to the hated city of Lankhmar. And this is where the real story begins.
"And in any case Lankhmar seemed no worse than any other place in Nehwon and more interesting than most. So they stayed there for a space, making it once more the headquarters of their adventurers."
THE JEWELS IN THE FOREST
Although the events of the previous story are not mentioned, it is implied this story happened after those, after the two adventurers had settled once again in Lankhmar.
Having pilfered from a Lord an old and cryptic letter that hints at a gigantic and apparently unprotected treasure in a tower hidden deep inside the forest, the two protagonists set off on their next adventure. It's hard to find a more D&D motivation than "huge cache of jewels in an abandoned tower, probably protected by an unknown menace," so it's easy to see why the adventures of Fafhrd and the Gray Mouser were always at the top of all the list of things that inspired D&D.
I'm sure the other contributors will write better analyses of this story (actually, they already have,) so I'll focus on a D&D-related piece of trivia hidden in this short story. Now, I don't know where I first heard about this, I guess it was some Old-School Revival blog years ago, but I haven't managed to find the original source. Still, I'm pretty sure I'm not making this up or, at least, I'm not the only one who has made this up.
There's a scene in this story where Fafhrd and the Gray Mouser are fighting some noble's henchmen (the same one from whom the Gray Mouser had stolen the letter,) and this happens:
"Fafhrd, his back to a great oak, had his broadsword out and was holding off two of Rannarsh's henchmen, who were attacking with their short weapons. It was a tight spot and the Northerner realized it. He knew that ancient sagas told of heroes who could best four or more men at swordplay. He also knew that such sagas were lies, providing the hero's opponents were reasonably competent.
And Rannarsh's men were veterans."
Used to other fantasy stories, where the hero slashes through hordes of enemies as if the were hay, this is an interesting shift in tone and style. But I want to focus on a specific sentence, the one about the ancient sagas and the four or more men. Because I think that sentence is the source of a D&D element: the name levels.
If any non-grognard is reading this, name levels (or "Titles") was a curiosity from the days of OD&D and Basic D&D. The official title of your class wasn't really "Magic-user, level 5" or something equally bureaucratic and bland, but Enchanter, which was the Title for 5th level magic-users. For example, this is the table for the Thief class, as found in p6 of the Expert Rulebook (1980.)
Now, I'm sure most people just said "my character is a level X-whatever, " but the Titles were kinda cool, and going from Apprentice to Master Thief probably felt great. Also, people like Titles.
Having explained that, we'll still have to go back even further, to the most distant origin of the RPGs, the Fantasy Supplement that appeared in Chainmail, the miniature wargame that Gygax and Jeff Perren designed. That supplement included many fantastic creatures we now take for granted, and also a lot of spells that would later appear in D&D, like Slow, Haste, Confusion, Fireball, etc. Being a wargame, there were no Player Characters, but there were special units, like Wizards and Heroes (and Superheroes)
HEROES (and Anti-heroes): included in this class are certain well-known knights, leaders of army contingents, and similar men. They have the fighting ability of four figures, the class being dependent on the arms and equipment of the Hero types themselves, who can range from Ligh Foot to Heavy Horse.
SUPER HEROES: Few and far between, these fellows are one-man armies! (Particularly when armed with magical weaponry.) They act as Hero-types in all cases, except they are about twice as powerful.
Some of you may have already realized what I'm trying to say, but if not, here's the table for the Fighter class as it appeared in the same Expert Rulebook, D&D 1st edition:
And that part about "they fight like four figures" was transferred directly to the old OD&D, since "fighting like a Hero" literally meant "fighting like four Men and having the Hits of four Men."
You can see that the title for level 4 and 8 are Hero and Super Hero, which are a reference to those Chainmail rules: Heroes have "the fighting ability of four figures, " and Super Heroes are "twice as powerful."
Now, reread that part I quoted from The Jewels in the Forest. This sentence to be specific:
"He knew that ancient sagas told of heroes who could best four or more men at swordplay."
"Hero" and "four (or more) men"? Well, isn't that a coincidence? Also, another coincidence, the title for a level 1 fighter is Veteran, as in "And Rannarsh's men were veterans." Translated into D&D-speak:
"He knew ancient rules that told of level 4 Fighters who could best four or more men at swordplay. He also knew that was horseshit, providing the opponents were reasonably competent and not level 0 mooks. And Rannarsh's men were level 1 Fighters."
Now, I'm not saying this short paragraph was the source of the whole idea of class levels but... it's a fascinating possibility. In any event, it seems clear that at least it did inspire the Titles and power-level (first via Chainmail) of Hero and Superhero.
Also, if anyone ever tells you that "heroic" adventure starts at level 12 or somesuch, or that level 4 characters are "low level," remind them that level 8 was already considered Superhero back then, before the Great Level Inflation.