Count Dooku, also known as Darth Tyranus, is a major antagonist in the Star Wars franchise. Once a Jedi Master, being the former Padawan of Yoda and Master of Qui-Gon Jinn, Dooku became dissatisfied with the Order and the corruption in the Galactic Republic, and later fell to the dark side of the Force.
After the death of Qui-Gon at the hands of Maul, Dooku became Darth Tyranus, the second apprentice of Palpatine, the Dark Lord of the Sith Darth Sidious. During the Clone Wars, Dooku served as the Count of Serenno and head of State of the Confederacy of Independent Systems, leading a pan-galactic secessionist movement against the Republic and the Jedi Order.
As a ruthless Sith Lord, Dooku secretly plotted to kill and overthrow his master, unaware that Sidious had his own plans to replace him with Anakin Skywalker. As such, when the right time came, Palpatine had orchestrated a lightsaber duel between Dooku and Anakin, who defeated and subsequently beheaded Dooku at Palpatine's urgings.