How to talk with Baal Namib at the Swamp Camp
Baal Namib is one of the Gurus in the Swamp Camp, and if you want to join the Brotherhood, you'll need to convince him that you are a loyal servant of the Sleeper.
In order to talk with him, you need to speak with Lester before, and ask him to take you at the alchemy lab, or somewhere else. As soon as you are far enough from the Baal, talk with Lester again and ask him how to get his master to talk with you. He will tell you what to do.
Now you can tell Lester that you've changed your mind, so he will go back to his original place. When you'll reach him at the gate, he will talk to you, and after the funny dialogue, Baal Namib will interact with you: tell him you want to serve the Sleeper.
After you have obtained the permission to join the camp from at least four Gurus, you will be able to talk with Cor Calom and become a Novice of the Brotherhood.
Also read the complete Gothic 1 guide.