Dooku, a Force-sensitive human male, was a Jedi Master that fell to the dark side of the Force and became the Dark Lord of the Sith Darth Tyranus during the final years of the Galactic Republic. After leaving the Jedi Order, he claimed the title Count of Serenno and, during the Clone Wars, served as Head of State of the Confederacy of Independent Systems. He was the second apprentice of Darth Sidious, the Dark Lord of the Sith whose plan to conquer the galaxy relied on Dooku leading a pan-galactic secessionist movement against the Republic. As such, Dooku immersed himself in the dark side and worked tirelessly to advance his master's plans, but ultimately forgot that treachery was the way of the Sith.

Born in 102 BBY during the Age of the Republic, Dooku learned the Jedi arts as the Padawan of Yoda, the legendary Grand Master of the Jedi Order. A political idealist, the corruption in the Galactic Senate—as well as the traditions of the Jedi—disillusioned him, causing Dooku to voluntarily leave the Order and return to his homeworld where he reclaimed his title and heritage as a nobleman. His former peers believed his decision was misguided but still held him in high esteem. Unknown to the Jedi, however, Dooku not only renounced their ways but his commitment to the light side of the Force as well. Having sworn his allegiance to Darth Sidious, Dooku became the Dark Lord's apprentice and adopted the secret name of Darth Tyranus. Throughout the first decade of Sheev Palpatine's chancellery, Dooku used his resources and charisma to recruit entire star systems to the Separatist cause. At the same time, he oversaw the development of two vast armies that would be pitted against each other in the conflict to come: the Separatist Droid Army, which consisted entirely of battle droids manufactured on Geonosis; and the Grand Army of the Republic, composed by clone troopers bred on Kamino and modeled on the bounty hunter Jango Fett.

After years of maneuvering the galaxy into a state of war, the Clone Wars began when the newly formed Republic Military invaded Geonosis in 22 BBY. As leader of the Separatists, Dooku delegated military authority to a cadre of subordinates personally trained by himself, including General Grievous and Asajj Ventress. While Grievous was trained only in the art of lightsaber combat, Ventress was strong with the Force, affording her the opportunity to learn the ways of the dark side from Dooku. The Sith, however, remained faithful to Darth Bane's Rule of Two; and as such, Dooku was compelled to betray his disciple after Sidious became suspicious of his own apprentice's intentions. Though he sought to replace Ventress with Savage Opress