The Nightbrothers were a tribal group of Zabraks that resided on Dathomir. Nightbrothers were trained to be highly skilled warriors who were subservient to the Nightsisters. During the Clone Wars, the Nightbrothers were led by Brother Viscus.
The Nightbrothers would only come together with the Nightsisters when it was time to mate or they were looking for one of them to become Count Dooku's apprentice. This happened to Darth Maul and Savage Oppress and to Asajj Ventress.