Count Dooku of Serenno was a legendary Jedi Master and the Count of Serenno. Later in life, Dooku fell to the dark side of the Force and became Darth Tyranus, the Sith apprentice of Darth Sidious, replacing Darth Maul and being replaced in turn by Darth Vader.
Dooku left the public eye and disappeared, not resurfacing until eight years later. By that time, he was no longer the Jedi Master that he once was, but a fully-fledged Dark Lord of the Sith.
After the death of Darth Maul, a Sith Lord who was under training for many years, Sidious had no time to mold another potential Sith into a perfect warrior, as the Battle of Naboo sparked the start of his climb through the Republic. Requiring another apprentice, he chose the maverick Jedi Master. Dooku fell to the dark side of the Force as a result of his own arrogance and pride, the former Jedi Master transformed into Sidious's Sith apprentice, Darth Tyranus. His murder of his friend Sifo-Dyas sealed his transition.
As Darth Tyranus, Dooku's first mission was to recruit a template for the Grand Army of the Republic's clone troopers to be used by Chancellor Palpatine and the Republic. In the end, the ex-Mandalorians Montross and Jango Fett faced each other. Fett triumphed over his rival and claimed the reward. He agreed to be cloned on the condition he received the first unit "unaltered".
With his plans in motion, Count Dooku reappeared on Raxus Prime, heavily criticizing the Republic and the Jedi Order for their decadence, hypocrisy and corruption. Dooku, a political mastermind, publicly called for entire systems to secede from the Republic and join the Separatist movement, culminating in the formation of the Confederacy of Independent Systems.
Darth Bane's Rule of Two prevented Tyranus from taking his own, bona-fide apprentice. Instead, Dooku acquired a couple of Dark Acolytes — Asajj Ventress and Quinlan Vos.
As the Clone Wars continued, his identity became more and more feared across the Galaxy. The Jedi suspected their former comrade was indeed a true Sith, but could not discern the identity of the elusive second Dark Lord. Darth Sidious, the true Sith Master, remained in the shadow. Then, like a dejarik master of old, the Dark Lords put in motion their final moves.
At the Battle of Coruscant, Dooku, with the help of General Grievous, "captured" his Master in an elaborate plan to be apprehended himself. They both knew that the only rescuers likely to arrive in time would be Kenobi and Skywalker. Despite Sidious's wish, Dooku tried to convince his Master to trade out Skywalker for Kenobi, citing that a Jedi of Kenobi's integrity would lend support to the New Order. Sidious disagreed and claimed that Kenobi was too indoctrinated by the Jedi way. So the plan stayed as originally conceived: Dooku would kill Kenobi, but be "defeated" by Anakin.
He would then, in captivity, turn on the CIS by surrendering to the Republic forces and claiming to have only recently learned of the extent of the atrocities committed by the CIS, and specifically General Grievous, who would be the perfect scapegoat on whom to hang all of the CIS's war crimes. He would later ostensibly emerge as Emperor Palpatine's right-hand man in the new Empire, with a newly turned Anakin Skywalker leading a new Sith Army of dark side Force-wielders to replace the Jedi Order and bring order to the Galaxy.
The day would not turn out that way, however. Against seemingly overwhelming odds, Anakin was eventually able to outmaneuver Dooku, overmatching Dooku and proceeded to cut off both of Dooku's hands, bringing the Count to his knees. Expecting his Master to intervene and save his life as Sidious had promised to do in the case of defeat, Dooku instead was shocked to see the Dark Lord relentlessly goading Anakin to deliver the deathblow, thus driving him one step closer to the dark side.
It was only then that Dooku finally came to realize the second deception: Palpatine never intended for the Count to survive, nor for him to be his apprentice; he was simply the tool used to create the CIS and make Sidious's plan truly possible. Anakin, holding two lightsaber blades to Dooku's throat, beheaded the helpless Count — driving one more dark side wedge into his soul.
Anakin Skywalker would then proceed to become Darth Vader, Darth Sidious's third, but certainly not last, apprentice. Luke would go on to serve the Dark Lord years later.
