Grande parte da história de fundo da decisão de Dookan de deixar o Jedi e se juntar ao Sith é explicada no romance Legends Darth Plagueis . Darth Plagueis e Darth Sidious tiveram inúmeras conversas com Dooku e Sifo-Dyas para influenciá-los.
Dookan ficou convencido de que a República precisava ser destruída e reconstruída para resolver seus problemas. Dooku sentiu que isso era necessário porque o Senado era ineficaz e o Conselho Jedi não estava disposto a liberar todo o poder da Força (ou seja, usar o lado sombrio) para consertar os problemas da República (ao contrário dos Sith). Dookan estava convencido de que ele poderia reconstruir a República trabalhando com Palpatine (ele ainda não sabia que Palpatine era um Lorde Sith e havia desenvolvido uma amizade política com ele) e os Sith. Além disso, ele sentiu que os Sith não eram um mal puro, mas apenas se aproximavam da Força de uma maneira diferente e menos restrita.
As citações relevantes de Darth Plagueis são as seguintes (eu ousou as partes particularmente importantes):
Dooku adopted a sly smile. “I understand politics, my friend. I know that you have to be circumspect about what you say and to whom. But that the disenfranchised worlds of the Outer Rim enjoy any support at all is largely due to you. You speak honestly and you champion the underprivileged, and you may be the only one capable of bringing the Republic back from the brink. Unless, of course, you have been lying to me all these years.”
Palpatine made light of the remark. “Perhaps a few lies of omission.”
“Those I am willing to forgive,” Dooku said, “whether or not we become partners in addition to being allies.”
Palpatine interlocked his hands. “It is an interesting notion. We would have to deepen our conversations, become completely honest with each other, bare our innermost thoughts and feelings to determine whether we truly share the same goals.”
“I’m being honest when I tell you that the Republic needs to be torn down and built up again from the ground up.”
“That is a tall order.”
“Tall, indeed.”
“It might require a civil war.”
“And how far from that are we now?” Dooku fell silent for a moment, then said, “The Senate grapples with trying to solve disputes the Jedi often see firsthand. What laws are enacted only follow from our having brought our lightsabers to bear.”
“It was the Jedi who pledged to support the Republic.”
“The Order’s place in this is a matter Sifo-Dyas and I have discussed endlessly,” Dooku snapped. “But the members of the Council are not similarly inclined. They are entrenched in archaic thinking, and slow to embrace change.” He paused, and adopted a sinister look. “Don’t let yourself be fooled, Palpatine. They see dark times ahead. In fact, they think of little else. That’s why they have allowed the Jedi to become involved in parochial conflicts like those at Galidraan, Yinchorr, and Baltizaar, which are like brush fires born of windblown embers from a massive blaze just beyond the horizon. But instead of actually rising up against the corruption in the Republic, perhaps disbanding the Senate entirely for a period of time, they have become fixated on prophecy. They await the coming of a prophesized redeemer who will bring balance to the Force and restore order.”
“A redeemer?” Palpatine stared at him in authentic surprise. “You’ve never alluded to this prophecy.”
“Nor would I now if I still thought of myself as loyal to the Order.”
“I never considered that the Force needed to be balanced.”
Dooku’s lip curled. “The Order interprets the prophecy to mean that the dark tide needs to be stemmed.”
“You don’t accept it?”
Dooku had an answer ready. “Here is the truth of it: the Jedi could fulfill the prophecy on their own, if they were willing to unleash the full powers of the Force.”
“The full powers of the Force,” Palpatine said. “I’m afraid you’ve lost me.”
Dooku blew out his breath. “Perhaps it’s something we can discuss in the future.”
“You’ve made your decision, then?” Dooku nodded. “If one more Jedi dies because of indolence on the part of the Republic and moral equivocation on the part of the Council, I will leave the Temple and refuse to look back.”
p. 293
[Palpatine] "...But if I am elected, and if you and I are going to begin to work together, it behooves us to give all appearances of being on opposite sides.”
Dooku folded his arms and stared. “Work together in what capacity?”
“That remains to be seen. But our common goal would be to return the Republic to what it once was by tearing it down.”
p. 344
[Dooku] “Because I suspect that Naboo was only the beginning—a kind of opening salvo. The Sith want to see the Republic brought down. Much as you and I do.”
Palpatine didn’t respond for a long moment. “But to ally with a Sith …”
“For many, they are the embodiment of pure evil, but the Council knows differently. What separates a Sith from a Jedi is the way each approaches the Force. The Jedi Order has placed limits on itself, but the Sith have never shied from incorporating the power of the dark side to accomplish their goals.”
“You wish to learn the secrets of the dark side?”
“I confess that I do.”
Palpatine restrained an impulse to reveal his true identity. Dooku was strong in the Force, and might simply be attempting to draw him out. On the other hand...
“Something tells me that this hidden Sith may eventually find his way to you,” he said at last. “And if and when he or she does, I hope that the alliance you forge will help us restore order to the galaxy.”
p. 365