Knights of the Round Uprising

Knights of the Round Uprising was a small scale conflict that took place in the earliest stages of the Zero Requiem, soon after Lelouch's ascension to the throne. This battle also acted as the debut of the Lancelot Albion, the final Knightmare piloted by Suzaku Kururugi in Code Geass R2.

Background
After Lelouch stops his parents' plan from unleashing the Ragnarök Connection across the world, Lelouch installed himself as the 99th Emperor of the Holy Britannian Empire, with Suzaku Kururugi as his personal knight: the Knight of Zero. However, some within the Empire refused to recognize Lelouch as their new Emperor and set out to usurp him from the throne. Most notable among them were the Knights of the Round, who (as it was revealed in the final volume of the Oz the Reflection O2 manga) were acting with the consent of Schneizel, Cornelia and Nunnally.

The Battle
A group of the previous Emperor's loyalists, led by the Knights of the Round, are en route to Pendragon, intent on removing Lelouch from power. Among this strike force was Bismarck Waldstein, Dorothea Ernst, Monica Krushevsky, and Gino Weinberg, along with each of the Knights' personal forces, with Bismarck in the lead. Bismarck announces how they swore absolute loyalty to Charles, Dorothea adding that they'd never recognize Lelouch as Emperor. As they approach, they are intercepted by Suzaku who, behind the controls of the Lancelot Albion and determined to protect Lelouch, unleashes the Albion's power and engages the entire force single-handed.

Suzaku immediately kills Dorothea and Monica using his Albion's new Super V.A.R.I.S. energy rifles. He then immediately destroys all of the participating Sutherlands as well, using a barrage of energy blasts from his unit's new Energy Wings. Gino then engages Suzaku with the Tristan, all the while trying to convince Suzaku to rejoin the Knights. Suzaku replies that Gino ought to join him instead, since he swore loyalty to Britannia. After inquiring whether Gino would prefer to be his and Britannia's ally or would rather become a rebel against Britannia, Suzaku disables the Tristan, crippling its Float System with his Slash Harkens and finally downing the unit with a spin-kick amplified by the Albion's Blaze Luminous. Though Gino was downed, he was spared, and the conflict of interest Suzaku sparked within him led Gino to eventually join the Black Knights.

Bismarck Waldstein then faces Suzaku with his Excalibur drawn, refusing to recognize Suzaku's title 'Knight of Zero'. He activates his Geass, enabling him to peer seconds into the future, claiming he only ever had to use this power against Marianne vi Britannia, the former Knight of Six. Reminded by Lelouch that the Zero Requiem hung in the balance, Suzaku forcefully activates the "live" Geass command that Lelouch had placed on him, in order to provide him an advantage. In a final clash with Bismarck, Suzaku ends up slicing the Galahad and even Excalibur itself in half in a vicious frontal attack. Bismarck's last word was "Marianne!" After Bismarck dies, the halves of the Excalibur fell in front of the downed Tristan. These, along with the unit itself, would later be recovered for the construction of the Tristan Divider.

Aftermath
After the battle ended, Lelouch announced that the clash acted as final proof of his standing as Emperor. With his position secure, Lelouch soon announces to the world that the Holy Britannian Empire will join the United Federation of Nations; to the shock of those who were watching the battle.