Forbidden lore is the secret lore of the demiplane of dread, having to do with the Mists, domains, darklords and the curses than bind them all together. There are very few people who know these things in-game. Three groups who are in the know are the Knights of the Shadows, the Fraternity of Shadows, and the Vistani. Among the PC's, the Darkonese party knows a lot more about the demiplane than the other group, which is fitting when their party includes a seeress and a KoS member, and their secret lair is an interplanar nexus.
A previous audience infodump pointed out a pattern to the vocabulary of each group:
"There's a pattern to the in-game references to Darklords, based on where you are getting your lore. The Vistani use the term 'Dark Master,' from the card of the same name in the Tarokka deck. The Knights of the Shadows use the term 'Dark Lord' (two words). The Fraternity of Shadows uses the term 'Dread Lord.'
"The terms are pretty interchangeable, even among these groups, but you will find a few sticklers. Some Knights avoid the Vistani term because they think it denotes a master/servant relationship, where the Vistani pay fealty or even overtly serve these masters. Yes, there's a lot of prejudiced stereotyping there, but Knights aren't perfect. As many of their younger generations have pointed out, 'Dark Lord' doesn't actually avoid this problem.
"The Fraternity of Shadows has only been hinted at by the Earth Weird, but its term 'Dread Lord' has also been adopted by some of the Knights...despite the fact that it WAS originally intended to express obeisance, or at least arms-length respect. Of course, the whole point of this is that it's all in how you say it. If you fall on your face and say, "What is thy bidding?" well then it doesn't matter if you follow that with dark or dread or lord or master."