Lord Umbriss was a Sith Lord belonging to the reconstituted Sith Empire, and they mentored Sadic as their apprentice. Sadic served as the Sith Lord's dedicated assassin for a number of years, but prior to 3643 BBY, during the Cold War era with the Galactic Republic, Umbriss drew the ire of another Sith Lord named Darth Angral. Because of Umbriss's actions, the significantly more powerful Darth Angral killed Umbriss, seizing their wealth and estate. Angral then claimed Sadic as his apprentice, and Sadic swore unwavering allegiance to his new Master.
The sole reference to Umbriss is found within the video game Star Wars: The Old Republic, specifically within an in-game Codex entry. The entry concerning their apprentice, Lord Sadic, recounts the death of Umbriss. Players controlling a Jedi Knight unlock this Codex entry upon defeating Sadic in the class mission titled "Showdown with Lord Sadic," which takes place on the planet Nar Shaddaa.