Zero Requiem

Zero Requiem (ゼロ・レクイエム, Zero Rekuiemu) is a plan orchestrated by Lelouch and carried out with the assistance of Suzaku and possibly C.C. to end all war and to usher an age of peace. The plan involved focusing all of the world's hatred on Lelouch, then have Suzaku disguised as Zero kill him, dissipating the hatred and allowing the world to focus on more important matters than fighting, such as reconstruction and helping others, thus facilitating peace. This is also Lelouch's own personal way of atoning for his sins, particularly the murder of Euphemia li Britannia and defamation of her name, of which the event closely resembles.

After having stopped the plan of Charles zi Britannia and Marianne vi Britannia on creating a new world, Lelouch announces to the world that he had killed Charles and is now the new emperor of Britannia with Suzaku as his "Knight of Zero" thus creating a confusion among those who had known their past conflicts. Zero then launches his plan on "destroying the world" starting from making the United Federation of Nations and the Black Knights as his enemy. Later on, the UFN and the Black Knights joined Schneizel in fighting against Lelouch. Throughout these fights it was made known that Lelouch was a dictator who was the enemy of the world--thus making Zero Requiem a complete success. It was never told to the viewers however until episode 25.

Having won the battle against Schneizel and the Black Knights, Lelouch arranges the public execution of The Black Knights' leaders and the United Federation of Nations representatives. When Suzaku, disguised as Zero, interrupts and kills Lelouch, the culmination of the Zero Requiem goes as they had planned. By directing all the hatred of the world onto him, Lelouch's death served to facilitate peace. He dies smiling, saying he is the one who both "destroyed the world and created it anew." Suzaku is left to play the role of Zero. Nunnally can only watch in shock. When Lelouch falls near her, she takes his hand and realizes that he has sacrificed himself on purpose to bring world peace. Lelouch dies in the arms of a crying Nunnally, who then succeeds her brother as Empress of Britannia.

Aside from Lelouch, Suzaku and C.C., several other key characters knew about Zero Requiem, including Jeremiah Gottwald, Lloyd Asplund, Cécile Croomy, Sayoko Shinozaki and Nina Einstein, while others such as Nunnally, Kallen, the core Black Knights members, and Cornelia only realized it during and after his execution.

Suzaku, as Zero, becomes Nunnally's protector in the following months, aided by Schneizel, who Lelouch ordered to specifically obey Zero. The aftermath is narrated by Kallen, who ends up forgiving Lelouch as she has realized Lelouch's true intentions were noble and explains how the world is much better off with all the energy being focused on reconstruction.

Conversation
As Suzaku (being Zero) killed Lelouch

"The punishment for what you have done shall be this then...you will live on, always wearing that mask, serving as a knight for justice and truth. You will no longer live your life as Suzaku Kururugi, you shall sacrifice the ordinary pleasures of your life in benefit of the world for eternity." - Lelouch

" This Geass I do solemnly accept." - Suzaku

(Lelouch fell to Nunally)

"Lelouch? Are you...?" - Nunally

(Lelouch is silent and looks out forward with a sad smile on his face)

(Nunally gasps and puts her hand on Lelouch)

(Nunally visionalizes Lelouch and Suzaku putting the plan in action)

"You mean...everything you've done until now? (Places Lelouch's hand to her cheek as tears fell from her eyes) Oh, big brother! I love you!" -Nunally

"Yes... I... I destroy... the world... and create it... anew." - Lelouch

Trivia

 * The idea of Zero Requiem is similar to the sacrifice of Treize Krushrenada from Gundam Wing to end all wars.
 * The scene between Suzaku stabbing Lelouch parallels Kamui and Fuuma killing each other to bring peace to the world in both TV version and Movie version of X/1999.
 * Sasuke from Naruto had a similar plan to have the 5 Great Shinobi Nations hate him to usher an era of peace, however, he does not intend to die for the sake of peace, but to live on as a continuing beacon of hatred, keeping all nations united. Naruto believes this plan is flawed since if Sasuke would die, the hatred would not die with him.
 * In Mutuality, it was revealed that after Lelouch, Suzaku and C.C. returned from the C's World, they stayed in Baghdad, Iraq where the Zero Requiem was formulated as a resolution.