One question that is bound to come up when starting a Lankhmar campaign is what to do with Fafhrd and the Gray Mouser? Sure, it’s fun to read about their exploits, but will they overshadow the player characters if you have them in your game? So what are the options?
Having the PC’s timeline coincide with the stories is a natural option. One decision you’ll have to make is when in the stories do you want the PC’s adventures to sync up with. What can be fun here is the chance to either relate to the players' events from the stories that the public would be aware of–you could even twist the events with rumors and mis-tellings–or to involve the players somehow in the stories even if only peripherally.
There are dangers here. If the PCs hear too much about Fafhrd and the Gray Mouser, their own adventures may become overshadowed. Also, by involving the PCs with the established stories, those storylines risk being altered. As a GM, you will need to decide how much change you are willing to allow.
On the other hand, you can use the news of the two to help build the world in your players’ minds. Perhaps the characters are tailing a cultist through the market when they overhear some gossip about Fafhrd and the Gray Mouser leaving Illik-Ving equipped with some mountaineering gear. Perhaps the gossipers even start to talk about glowing mountain gems and the dangers of Ice Gnomes.
It can be tempting to have the PCs meet up with Fafhrd and the Gray Mouser. There is nothing wrong with this, but questions do arise. Are they peers or are the PCs their juniors? What role will the twain play? How does everyone relate? Does a meeting make the game more enjoyable for the players? Does it add to the story?
Another way to run things is to never have them meet. Simple enough. Hopefully, the players do not decide to meta-game and force a meeting!
One potentially entertaining option would be to have the PCs always coming close, but never quite meeting Fafhrd and the Gray Mouser. There could be chance sightings across a market with plenty of pedestrians in the way. They might be spotted jumping from roof to roof one evening. On another occasion, they could be seen aboard a ship landing in the royal docks away from public access. And so on and so forth. Given enough permutations, this could become a running gag and a way to lighten things up on any given night!
At the end of book six, Fafhrd and the Gray Mouser sail off on a mission to Rime Isle where they end up in semi-retirement and mostly living the lives of responsible and monogamous men, even if the Lankhmarian public doesn’t want to part with its two entertaining heroes.
Rime Isle is sufficiently out of the way–never mind being a mythical island that most sailors swear does not exist–that the two are also out of the way of our new heroes, the player characters, and their adventures. This is a most convenient option that not only has Fafhrd and the Gray Mouser out of the way but also has the bulk of the story history already established. From here, everything the PCs do is new history.
Say it isn’t so, but it is a viable option. You could just take the world, pretend Fafhrd and the Gray Mouser never existed, and replace them with the PCs. You could either run entirely new adventures with the world as just a background or run the PCs through some of the Fafhrd and Gray Mouser stories and see what they do with them.
Whatever you do, hopefully, you’ll find something that will work for your game and makes a fun time for you and your players!