During the final days of the Great Hyperspace War, the Sith Emperor and his closest followers remained upon a world known as Nathema and did not engage the Republic. It is clear from the records that Nathema is not the original name of the world, but why it was changed and why he was there has been carefully purged from all records.
What is known is that following the flight of Naga Sadow, the Emperor came to Korriban, gathered the remaining lords of the Sith and took them back with him to Nathema. There they conducted a ritual that extended the Emperor's life at the cost of the lives of more than eight thousand Sith Lords. The Ritual of Nathema is celebrated among Imperial scholars as a rare and amazing coming together of Sith for the good of the Empire.
The Emperor then gathered the younger generation of Sith, promoted new lords and promised them revenge on the Republic for its attempted genocide of the Sith people. Aboard a small fleet, the Sith Empire vanished into unknown space, leaving no trace for the Republic to find.