Count Dooku, a Force-sensitive Serennian human male, was a Jedi Master who 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 served as Head of State of the Confederacy of Independent Systems during the Clone Wars. 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 and his master’s plans, but ultimately forgot that treachery was the way of the Sith, while also being unaware that he was just a pawn of his master. His former peers believed his decision was misguided but still held him in high esteem. However the Count of Serenno began to secretly conspire with Darth Sidious, before abandoning his commitment to the light side of the Force after the death of Jinn by Darth Maul, becoming the Dark Lord’s next apprentice and adopting the title of Tyranus.
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 and later Quinlan Vos, Dooku ultimately remained beholden to his master’s will, which would eventually be his undoing. During the ensuing Battle of Coruscant, he was confronted and defeated by the Jedi Knight Anakin Skywalker, who summarily executed the Count via beheading at the Chancellor’s behest.
In the days following Dooku’s death, Palpatine revealed to the Jedi that he was, in fact, Darth Sidious, who betrayed his apprentice in order to replace him with the younger and more powerful Skywalker. With the subsequent fall of the Jedi Order, Sidious consolidated his power upon declaring himself Emperor of the Galactic Empire, which Tyranus had an instrumental role in establishing. As for Skywalker, he succeeded the late Darth Tyranus as Sidious’ third and last apprentice, resulting in his transformation into Darth Vader and proving his slain victim’s words of sensing fear, hate and anger.