All of the Sith Empire’s most powerful leaders from the post-KOTFE expansions were all created in 2011 for SWTOR’s vanilla content cycle in one way or another.
While Darth Malgus was the posterboy of the game at launch and characters like Acina and Vowrawn featured heavily in important Storylines and Quests, the two most mysterious are Darth Anathel and Xarion.

Most players probably recognise Darth Anathel as “that guy that my Inquisitor destroyed after Corellia so I could rejoin the Empire” and Darth Xarion as “that masked guy that is Darth Rivix’s boss”, however BOTH of these characters were created in name in 2011 for the Grade 3 Lightside Diplomacy Crew Mission: Blind Hatred.

The warlords Anathel and Xarion are battling in the Stygian Caldera, disrupting valuable trade. Your companion could persuade them to take their feud elsewhere.

Blind Hatred mission description.

While the results of the ‘feuding warlords’ is unknown, we can be sure for certain that when your player character Inquisitor’s choose to kill Anathel and take over his seat on the Dark Council in the Empire during Onslaught, Darth Xarion was probably thrilled as you have most likely finalised the feud between the two. If it was ever still going, as over ten years of ingame time had passed between the beginning of SWTOR and Onslaught.