Darth Vatrix was a Sith of unknown gender and species during the time of the Hidden War.
During the Dark Times and the years after the defeat of the Empire, Vatrix remained hidden on Dathomir until he learned of Rovaniss' Sith Empire. Vatrix made his way to Korriban where it is said that he pledged himself to Rovaniss, the Sith Empire becoming one of the only living beings to see Vatrix's real self.
Vatrix became a General during the Hidden War with Soru's Jedi Order, leading by fear rather than example. Even many of the more powerful Sith Lords such as Darth Gerret found Vatrix's presence to be unnerving