Darth Subeth was a very powerful Sith Lord during the 70s, 60s, and 50s BBY. He had originally been a Jedi, but he turned to the dark side and vowed to destroy all the Jedi, or if he couldn't accomplish this, at least train another so the apprentice could destroy the Jedi. For the duration of the 70s BBY and the first half of the 60s as well, none of the galaxy knew Darth Subeth even existed. During that time, he was training and building an army. This army became known as the Hidden Sith Army because Darth Subeth wanted the army to be hidden from the galaxy until it became big enough for conquest of the galaxy. And big it did become. By the time of the First Battle of Ris'danon, the Hidden Sith Army had 150 Dreadnaughts, many smaller support ships, as well as about 850,000 ground soldiers