Dark Underlord

The Dark Underlord was a Dark Lord of the Sith during the New Sith Wars. His origins were a mystery, but theories about his identity ranged from him having being summoned from the realm of Chaos to him being the spirit of the Lettow General Xendor.

Biography
During the first quarter of the New Sith Wars, the Dark Underlord was known as a bloody marauder earning the ire of the Galactic Republic. As the leader of the Sith forces, he was able to consolidate a large group of elite Sith warriors known as the Black Knights. Roughly 1,750 BBY, the Dark Underlord and his Black Knights had set up base on the planet Malrev IV. The planet contained a powerful Sith Temple which magnified the use of the dark side of the Force. The temple itself may have been constructed by the Dark Underlord to help counteract the Jedi army; however, it's possible the temple was built before the Dark Underlord's time.

In any case, the Jedi General Murrtaggh, a perennial enemy of the Dark Underlord, prepared to eliminate the dangerous Sith Lord while his forces were stationed there. Unknown to the Dark Underlord, Murrtaggh paid a large group of Mandalorian mercenaries to stage a diversionary attack on the Black Knights while Murrtaggh prepared to sneak behind the enemy lines to assassinate the Sith leader.

As the battle commenced, the Dark Lord sent his foremost general to lead his forces against the Mandalorians. During the chaos of the battle, Murrtaggh executed his plan and infiltrated the Dark Underlord's fortress. As the two engaged in battle Murrtaggh fell to the dark side, striking down the Dark Lord of the Sith.

Talents
The Dark Underlord was one of the few Sith with enough power to actively wage war against the Jedi. The Sith Lord was a practitioner of Jar'Kai, wielding a pair of Sith swords as weapons, despite the fact that they were heavily dated by the time of his reign.

Behind the scenes

 * Abel Peña's article Evil Never Dies: The Sith Dynasties states that the Underlord became a Sith leader some 250 years after Darth Ruin's death (who lived circa 2,000 BBY) and that "he was a bloody marauder of the first quarter of the New Sith Wars." His date of death is currently unknown.
 * The image of the Dark Underlord originally appeared in the Dark Side Sourcebook as a "Dark Side Spirit".
 * A picture of an Unnamed Zeltron Sith General in The History of the Mandalorians was initially identified by many fans as being the Dark Underlord, but Evil Never Dies: The Sith Dynasties established that character as one of the Underlord's generals.
 * The idea of the Underlord being a sort of dark side phantom came from Peña's original idea for Blackhole.