Naga Sadow is a character from the Star Wars Expanded Universe (also known as Star Wars Legends.)
Sadow was a Dark Lord of the Sith who was the leader of the Sith Empire during the Great Hyperspace War. He was the successor to Marka Ragnos as Dark Lord of the Sith and a fierce rival of Ludo Kressh. After capturing a pair of explorers named Gav and Jori Daragon, he trained Gav as his apprentice and built up the Sith Empire's military forces to the point that they were formidable enough to conquer the Republic.
Even though Sadow's efforts to try and conquer the Republic were successful at first, he encountered setbacks when he was betrayed by his apprentice and later encountered a revived Ludo Kressh, who had built up an army of his own. After his forces were decimated by Kressh and later, the Republic, Sadow was forced into exile on Yavin 4, where he built several temples and practiced alchemy for a number of years before willing putting himself in suspended animation.
600 years later, Sadow was awoken by the fallen Jedi Freedon Nadd. Sadow taught Nadd the ways of the Sith before being killed by him.