Volo's Student's Guide to
Lankhmar
Lankhmar is technically a city on the world of Nehwon, but it's an important enough city that off-planars usually refer to the whole plane by its name. It is protected by two great heroes Fafhrd and the Gray Mouser (who were invited to join the faculty, but, you know, copyright stuff).
The city is rife with corruption, and is labyrinthine (but since you kids go to school in Sigil, you probably won't be bothered by it). It's ruled by it's Thieves' Guild and criminal underbelly, but, well, you kids are from Sigil... You know what? Unless you actually want to meet the Gray Mouser, you might as well adventure in Sigil...