Dooku is a Jedi master who crossed over to the dark side of the Force and became the dark Sith Lord Darth Tyrannus. Born on the planet Sereno, he inherited great wealth and the noble title of Count. Dooku was taken from the Jedi Order as a child and became a student of Yoda. As a Jedi Knight, he took Qui-Gon Jin as his first Padawan. Dooku is a respected teacher in the Jedi Temple and one of the most famous in the Galaxy with the lightsaber. Only Masters Yoda and Mace Windu are considered equal to him.
Dooku spent nearly seventy years of his life as a Jedi, but Galidran's fatal battle shook his faith in the Jedi Order and the Galactic Republic. He came under the influence of Senator Nabu Palpatine and left the Order. After learning that Senator Palpatine was in fact the Sith Lord Darth Sidious, Dooku turned to the dark side of the Force and also became a dark Sith Lord, taking the place of Darth Maul as his student. His new master gave him the Sith name Darth Tyrannus.
Dooku regained his birth title of Count of Sereno and his vast wealth, and he and Sidious devised a plan to create a war in the Galaxy to raise the Sith to power. He built ties between corporations and planets dissatisfied with the Republic, and became the leader of the separatist movement, which became the Confederation of Independent Systems. Thousands of systems hoisted the Dooku flag and withdrew from the Galactic Republic.
As the Confederate's political leader, Dooku led it against the Republic in a conflict that later became known as the Clone Wars. He recruited and trained four of his best warriors to lead the separatist army of droids: the Jen'dai Durge bounty hunter, his protégé and dark schoolgirl Assage Ventres, General Sevrance Tan, and General Greaves, the notorious cyborg, the killer of Jedi and Droid Guide.
The culmination of the war was a great battle at Coruscant, the capital of the Republic. In a duel on the separatist ship Invisible Hand, the Jedi Knight Anakin Skywalker defeats Dooku and executes the mutilated Sith, instigated by Chancellor Palpatine, the Sith teacher of Tyrannus. He is quickly replaced by the man who kills him, Anakin Skywalker, who soon turns to the dark side and becomes the fearsome Darth Vader.