Tormidan's past is known to very few, if any, beings in the galaxy. He has gone by many names and aliases, but he was born Galen Silver, and raised among the Jedi. He was powerful in the Force and considered wise by many who knew him. He believed wholly in the Jedi way, preferring peaceful words first and a lightsaber as a last resort, unlike many Jedi of the time. Always, he saw the best in things and people, and tried to bring it out when they could not. As was the case with most Jedi, he was assigned a master as a padawan, from whom he learned much and respected. Eventually, he passed the Trials and became a full-fledged Jedi Knight. All the while, though, he had come to believe that the Jedi were hiding something from him. And eventually, his suspicions proved correct.
Eventually, Galen resolved to search the Archives for information. He knew nothing of his origins, who his family were, where he came from...the Archives would surely tell him. What he found there changed everything he thought he knew. He was the last living heir of an ancient Sith bloodline going back thousands of years to the first Sith Empire. A desire rose up in him to learn more about his heritage, taking Sith records and holocrons when the Jedi weren't looking. He studied them, practiced them, until he reached a point where he was a Jedi no longer, but a Sith. He decided that he no longer had a future with the Jedi and left his life behind. The Order never knew what truly happened to him, as he simply vanished.
With his newfound power and allegiance, Galen decided that it was time to make use of it. He forsook the name he had once gone by, adopting the name Lord Void. He gathered followers from across the galaxy and began to take part in a number of criminal activities, building up his power base and sending spies throughout the galaxy, so that there were few things that happened that he did not know about. The Jedi and the Republic took note of this, not to mention Bane's line of Sith, the "true Sith", as they called themselves. He was thought killed many times, taking on various new identities, most famously Darth Kaar, eventually sticking with his current name: Tormidan.
Agent of Sidious
Eventually, the Sith Lord known as Darth Sidious took notice of the self-proclaimed Sith Lord, seeking to contact him. The two reached a mutually beneficial agreement: if Tormidan were to aid Sidious in the destruction of the Jedi Order as the time for their elimination grew closer, as well as any other "Sith" that jeopardized his plans, Sidious would allow his power base and criminal operations to survive and prosper. And so, at the height of the Clone Wars, Tormidan continues to build his power and eliminate any threats to the future victory of the Sith.