“What if I told you that the Republic was now under the control of the Dark Lord of the Sith? [...] The dark side of the Force has clouded their vision, my friend. Hundreds of senators are now under the influence of a Sith Lord called Darth Sidious.”
Count Dooku , also known as Darth Tyranus, is a major antagonist in the Star Wars franchise. Once the Jedi Knight and former apprentice of Jedi Master Yoda and Master of Qui-Gon Jinn, Dooku fell to the Dark Side, becoming the second apprentice of Darth Sidious and head of State of the Confederacy of Independent Systems after Qui-Gon's death at the hands of Darth Maul.
Dooku secretly plotted to kill and overthrow his master, unaware that he had plans of his own to replace Dooku with Anakin Skywalker. As such, when the right time came, Sidious had Dooku duel Anakin, who defeated and subsequently beheaded him at the Dark Lord's urgings.
He is the main antagonist of Attack of the Clones, a minor antagonist in Revenge of the Sith, and the main antagonist of the Star Wars: The Clone Wars film and television series.
In the films, he was portrayed by Christopher Lee and voiced by Corey Burton in The Clone Wars series.
Trivia
The scene in Revenge of the Sith, where Palpatine, when he was pretending to be a positive of the Separatist Alliance, orders Anakin to kill Dooku mirrors the scene in Return of The Jedi, the last film in the original trilogy, where he pressures Anakin's own son Luke to kill Anakin (who was now Darth Vader). However, the end-results are different, as Anakin killing Dooku is what would have happened if Luke gave in to the dark side and killed Vader.