The Nightbrothers are a clan of all-male Dathomirian Zabraks. They originated from the Zabrak home system Iridonia, and were cross-bred with the female Dathomirian Witches known as the Nightsisters. They are slaves to the Nightsisters, and most are used by them simply for labour.
Nightbrothers spend most of their time learning to fight with melee weapons as well as their bare hands. A select few are chosen by the Nightsisters to wield powerful weapons enchanted by Dathomiri Witch Sorcery, providing they are a capable enough warrior. Occasionally, an exceptionally gifted Nightbrother will be chosen to be imbued with dark energies, making him grow a foot taller, develop a broad chest, long horns, and gain a massive increase in muscle mass and general power. Savage Opress was one of these select few.
The infamous and powerful Sith Lord Darth Maul was born a Nightbrother, and shared their distinctive tattoos applied to their faces by Nightsisters. Savage Opress was Darth Maul's biological brother.