He fell under the influence of Supreme Chancellor Palpatine of Naboo, and left the Jedi Order. After learning that Palpatine was secretly the Sith Lord Darth Sidious, Dooku turned to the dark side and became a Dark Lord of the Sith himself, replacing Darth Maul as Sidious's second apprentice.
Masters. Sifo-Dyas was a human male Jedi Master from Minashee who commissioned the Kaminoans' creation of the Grand Army of the Republic over a decade before the Clone Wars.
Dooku did go sith shortly after battle of naboo in 32 bby. Sidious needed new apprentice after death of darth maul and tooked dooku under his wing and named him darth tyranus. Reason he chose dooku as his apprentice was because dookus form 2 light saber skills and power in the force.
Bottom line, Dooku never considered himself a Sith. And he wasn't; he was a man looking to change the galaxy in a way the Jedi were unwilling to try. And that is why he joined the Sith and became Darth Sidious' apprentice, Darth Tyrannus. When Dooku became Sidious's apprentice, he was already an elderly man.
Anakin gives in to his hatred of Dooku and uses his anger to overpower him, severing both of Dooku's hands and leaving him helpless. Palpatine then orders Anakin to execute Dooku on the spot, thus betraying Tyranus. After initial hesitation, Anakin decapitates Dooku, much to his horror of the betrayal by his master.
Anakin regrets this decision, and Palpatine reasons with him saying that Dooku was too dangerous.
We see this when Dooku confronts Obi-Wan and asks him for help destroying the Sith. He's telling the truth. He does want to destroy the Sith, he just wants to destroy the Jedi as well and enforce a government tailored to his particular ideals. "You must join me, and together we will destroy the Sith!"
Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth.
His best friend had been given a gift that could have allowed the Jedi to help countless people, but Yoda refused to act. As a Jedi, Dooku couldn't stand the thought that his best friend was suffering without a purpose, so he hated Yoda and the Jedi Order for that. In the end, though, Yoda was right.
Darth Maul
Even though they were both trained by Palpatine, Dooku and his predecessor, Maul have been enemies every since his mind was restored by Mother Talzin. This is because the two hated each other. Dooku despises Maul for being a threat to him and Maul hated Dooku for being his replacement.
Though he saw Anakin's potential, Yoda could not ignore the danger inherent in Anakin's unrestrained emotional attachments. To teach Anakin responsibility, Yoda assigned Skywalker a Padawan learner: Ahsoka Tano. Yoda knew there would come a time when Anakin had to let go of Ahsoka.
Mission to Mathas. Padawan Windu was trained in the ways of the Force by Jedi Master Cyslin Myr. Following his early trials as a youngling, Windu attained the status of Padawan by becoming Jedi Master Cyslin Myr's apprentice.
N'Kata Del Gormo was a Force-sensitive male Hysalrian Jedi Master who lived during the times of the Galactic Republic. According to legend, he found and trained Yoda and a Force-sensitive Human friend.
A member of the Jedi Council before the blockade of Naboo, Sifo-Dyas believed the galaxy would soon be plunged into war, and agitated for the Republic to create an army for its defense.
Billaba became Windu's Padawan and trained under the Jedi Master in the ways of the Force. During her training, Windu allowed her to study Form VII variant known as Vaapad. She eventually became a Jedi Knight, renowned for her skills with a lightsaber and favored Form III when teaching other Jedi.
Sometime after, Sifo-Dyas and Dooku were present at the Senate Building and met Senator Palpatine of Naboo, in reality, Damask's Sith apprentice (and Dooku's future master) Darth Sidious.
The decision to let the Sith Lord escape would ultimately lead to many more deaths throughout the Clone Wars. Yoda had to shoulder that decision for years. However, he ultimately understood the importance of life, and the power he could have used to defeat Dooku would have been much better used to preserve two lives.
In short, he thought that the Jedi were becoming too subservient to the Republic and the Senate. That was on particular display in "Choices," when he and Master Windu were working as partners.
At the time, Lucas said the name “sort of appeared in my head one day. I had lots of Darth this and Darth that, and Dark Lord of the Sith. The early name was actually Dark Water. Then I added lots of last names, Vaders and Wilsons and Smiths, and I just came up with the combination of Darth and Vader.”
He then orders Ren to bring Rey (Daisy Ridley) to him after she refuses to disclose the map to Luke. After Ren is defeated in a lightsaber duel with Rey, Snoke orders Hux to bring Ren to him to complete his training.
You may have noticed that Snoke does not have the “Darth” title like the Dark Lords of the Sith do. He was never given one because he isn't technically a Sith Lord.
In legends, Dooku actually explains why he hates Anakin: He could feel Yoda's touch still lingering on the edges like a distant echo. Vividly his mind went back to their last meeting, on Geonosis: swords drawn at last, and finally equal.
As a result, Jango ventured to the moons of Bogden, where he was recruited by the Sith Lord Darth Tyranus—the Sith identity of Count Dooku—to be the genetic template for the clone army created by the cloners of the planet Kamino.
Master Yaddle Was Dooku's Sacrifice To Become A Sith
The second part of Star Wars: Tales of the Jedi episode 4 is set shortly after Qui-Gon Jinn's death at the hands of Darth Maul, and it reveals the moment Dooku became a Sith Lord.