Truth be told, you probably have enough information in the Forgotten Realms Campaign Setting to pass through or conduct simple quests with regard to most of the evil groups of Faerun. All of the organizations in this book are mentioned, with varying detail, in the campaign. They are also described in Forgotten Realms novels, so if you have read a great deal of those, you might not need this accessory.
If you want to get into real detail with the powers of darkness and haven't read that much fiction, then this is the book to have. Even if you have done your homework, there's quite a lot of original material in here. As an example, if you'd like to actually visit the City of Shade, Lord of Darkness contains a wealth of information about how you would describe it. The campaign provides more simple demographics and locations than in-depth detail, so this is most welcome. There are also new encounters, weapons, and spells for the organizations, as well as some pre-made dungeons to explore. Specific points of interest in lands held by the enemy are also noted, as well as details on particular leaders of each order.
In all, every major evil organization is covered, as well as a host of lesser ones. Lords of Darkness has enough info about how they operate and what they look like to give a DM totally unacquainted with the FR setting the capability to correctly portray enemies and guide adventurers through enemy territory.