The Necrarchs are the most learned of Vampires, more skilled at sorcery than with the blade. These skills have come at a price, however. The appearance of the Necrarchs has grown so hideous and unnatural that an ordinary man cannot bear to look upon them, and many run screaming at the sight. When their great library was burned to the ground by fearful mortals, many Necrarchs remained to burn with it, so strong was their devotion to knowledge.
Since the removal of the Strygos from the Vortex campaign, they are the only Vampire Counts faction on the map. As such they are the only faction Arkhan gets bonus relations with and should be made an ally if they survive the early game.
On the other hand, as Settra, this is a faction that you should be able to crush quickly and easily once you get past his initial foes.
W'soran is the founder of the bloodline.
That this faction is referred to as a Brotherhood is ironic given that each Necrarch is notoriously paranoid of another Necrarch Vampire stealing their knowledge and are generally mistrustful of others of their kind.