Malachor III was a member of the Sith Empire long before the Cold War and even before the Mandalorian Wars; it was the burial ground of Terrak Morrhage, a Human male Sith Lord of the resurgent Sith Empire, dead centuries before the Cold War. His spirit remained dormant until it was awakened by a Jedi expedition composed of Parkanas Tark, Yuon Par, Duras Fain, Sidonie Garen, Eriz Vossan and Cin Tykan. Morrhage's spirit played with the emotions of the young Jedi and drove them insane. Having escaped the planet, the Jedi reported their discovery to the Jedi Council, which declared the planet as "barren."
Darth Thanaton, after he had supposedly killed Darth Zash's apprentice, left for Malachor III, where he had business to attend to.
- Star Wars: The Old Republic Encyclopedia