Darth Havok

Darth Havok, who was known as Eshkar Niin when he was a Imperial Knight, trained Antares Draco into a Imperial Knight. Sometime later, Niin, killed the wife of the Emperor, Roan Fel and mother of Princess Marasiah. Antares was ordered to kill him and apparently tracked Niin down for a duel with Antares winning and supposedly killing him. However, somehow, Niin survived.
 
Sometime later, Niin turned to the dark side and became a follower of Darth Krayt’s One Sith. He abandoned his former name and became known as Darth Havok. 
 
When Princess Marasiah was captured, he was ordered by Darth Wyyrlok  to take her to Korriban and interrogate her. While on Korriban, Havok tortured her, but she refused to submit. It was then that  Havok revealed to her that he was Eshkar Niin. She tried to retaliate with the Force, but was no match for Darth Havok.
 
Sometime later, Antares traveled to Korriban disguised as a Sith along with Ganner Kreig and Shado Vao to rescue Princess Marasiah. When she found her, she revealed that Eshkar Niin was still alive. As the Sith closed in, Antares decided to make a stand against the Sith and told Ganner and Shado to leave with the Princess. After taking on numerous Sith, Darth Havok appeared before him. As they dueled, Havok told Antares that always knew there was darkness inside of him and told him to join the Sith. Draco refused. As the battle went on, both Havok and Draco could sense the presence of Darth Krayt. Havok gave Draco one last chance to join him, but again Draco refused. Havok then struck Draco with Force Lighting, seemingly killing him.