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. Lelouch then launches his plan on "destroying the world" starting from making the United Federation of Nations and the Black Knights 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. On the verge of death, Lelouch smilies, saying he is the one who both "destroyed the world and created it anew." With the Zero Requiem complete, Suzaku weeps silently as he listens to Lelouch give his final order and punishment; he tells Suzaku that he is dead from this day on, forced to exist as Zero and sacrifice his happiness for the sake of the world, an order Suzaku accepts 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, Rakshata Chawla, Sayoko Shinozaki and Nina Einstein, while others such as Nunnally , Kallen, Cornelia, Ohgi and Kaguya 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.
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 destroyed... the world... and create it... anew." - Lelouch
- 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.