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, after Lelouch asssumes Britannian throne from his father from episode 22 to episode 25 of the second season.

Overview
The plan involved focusing all of the world's hatred on Lelouch, then have Suzaku disguised as Zero to 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.

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.

Lelouch as Emperor of Britannia
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.

He then quickly abolished many policies that grew during Charles' era. These included the elimination of the aristocratic system, serious penalties for financial conglomerates engaging in or a part of illegal or illicit activities, and the abolition of the Number-Area system, thus liberating them. This led to a rift among Britannian citizens with discontent among ex-nobles and approval among the commoners.

This also leads the Imperial family (including Nunnally) oppose his regime and attempt to dethrone him through force.

Knights of the Round Uprising
Shortly after Lelouch assume the throne, the Knights of the Round, being loyal to Charles, staged an uprising against him. Lelouch see this as an opportunity to show his power to the world, only sending Suzaku to repel it and broadcast the battle worldwide. And as he expected, Suzaku single-handedly stopped the uprising.

After the battle, Lelouch announced that he is the true ruler of Britannia and Britannia would join United Federation of Nations, much to everyone shock.

UFN Conference
Lelouch announced that he will follow every rules of UFN and promised that he will not accompanied by Suzaku nor any generals during the conference. However, he decided that the site of the negotiation should be in in Ashford Academy, Japan, as it is a neutral site standing between UFN and Britannian Empire, which UFN agrees

However, in the middle of conference, while everyone is still discussing whether to allow Britannia to join UFN or not, Lelouch break the agreement by calling Suzaku and his Lancelot Albion into the conference room and Britannian army surrounding the academy, kidnap the world leaders, thus making him the enemy of the world, with people seeing him as a ruthless dictator.

Battle of Mt. Fuji
Meanwhile during the conference, the Imperial Family, led by Schneizel el Britannia, destroyed the capital Pendragon with F.L.E.I.J.A. warhead deployed from Damocles, thus declaring war against Lelouch.

Schniezel proceed to face Lelouch's forces at Mt. Fuji using Damocles as his flagship and convince Lelouch to surrender, as his F.L.E.I.J.A. warheads could easily wipe out Lelouch's forces. But Lelouch instead used his hostage (the UFN world leaders) in his flagship, preventing Schneizel forces from using F.L.E.I.J.A. warheads to end the battle sooner.

The battle eventually ended with Lelouch's victory, after he took the Damocles Key from Nunnally and forcing everyone and whole world to surrender. Meanwhile, Suzaku faked his death in the battle and prepare himself for the final phase of the plan.

Death of Lelouch
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 in a victory parade in recaptured Japan. The truth is, the parade only serves as stage for the final phase of Zero Requiem.

Suzaku, disguised as Zero, then interrupts the parade 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.

Aftermath
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 solving hunger and poverty. Lelouch then become a hate symbol with all sorts of hateful and evil deeds blamed on him. Kallen herself now lives with her mother and returns to her normal life as Ashford academy student.

Suzaku now lives the remainder of his life as Zero, becoming Nunnally's protector in the following months, aided by Schneizel, who Lelouch ordered to specifically obey Zero.

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 destroyed... the world... and create it... anew." - Lelouch

Trivia

 * 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.