Count Dooku was a Sith Lord who led the Separatist movement during the Clone Wars. Dooku defected to the dark side after growing disgusted with the Old Republic. As a Jedi Knight, Dooku realized that the Jedi Order had become nothing more than a bodyguard operation for corrupt Republic leaders.
Dooku eventually came to study under Darth Sidious, who named him Darth Tyranus.
An influential and charismatic man, Dooku helped convince several star systems to join the Separatists, an organization bent on tearing down the Republic, as well as the Jedi Order.