Lanbril comes out of the tradition of fictional thieves’ guilds such as Leiber’s Lankhmar and Cervantes’ Rinconete y Cortadillo. His cult can be thought of as an approach to organized crime throughout southern Genertela. Think the triads, vorovskoy mir, and so on.
There certainly are a lot of them in the Holy Country. I’ve heard claims that there are more than 2000 initiates of Lanbril in Nochet alone, and that the total number of cultists in the Holy Country might be six or seven times that number, maybe more. Of course, like everything with Lanbril, these numbers might be exaggerated or might understate them!
Banditry, highwaymen, robbery, protection rackets, extortion, pick pockets, fences, smugglers – all of this is Lanbril’s area of action. City and countryside, you can find Lanbril operating in both.
I’ve always had a soft spot for the Faceless God. One of my first long-lasting RQ characters was a Lanbril cultist who worked his way up to being a Master Thief. A coward, despite being a very skilled swordsman, he stole from Duke Raus and ended up hiding a few seasons in the Big Rubble as he tried to fence his goods. He robbed the Cradle (well stole some stuff from it) before fleeing from Orlanthi and Lunars, and ended up involuntarily aiding the Unicorn Riders in their high priestess’ heroquest.
And not to be flippant, but to be a “made man” in most criminal organizations, you cannot have any other loyalties. Being an initiate of another god is incompatible with being an initiate of Lanbril.
Lanbril is Neutral towards Eurmal, and just about everyone else, although he dislikes BG, Dayzatar, LM, and a few others. He’s actually friendly towards Orlanth.
Nochet has plenty of parallels with the Golden Age Seville, and is a great setting for a campaign of thieves, sailors, scribes, and the like. Riches and exotic peoples from across the world come into Nochet, and you can’t blame Lanbril for trying to take his fair share of it!
And yes, there is a Thieves’ Argot – like Fenya, Gaunersprache, etc. It is used so that cult members can communicate without fear of being understood by non-criminals.