Fall of the Royal Capital

Siege of the Royal Capital was the besieging of the Royal Capital of the Re-Estize Kingdom by the forces of the Sorcerer Kingdom during the war between the two nations.

Background
During their war with the Re-Estize Kingdom, the Sorcerer Kingdom knew the key to a swift victory was capturing the Royal Capital. To this end, they centered their entire invasion plan around surrounding and isolating the city in preparation of besieging it. They thus covertly conquered the Kingdom's eastern and northern regions. By the time the Kingdom figured out what was going on it was already too late, as the Sorcerer Kingdom's forces had begun marching on the capital.

As Sorcerer Kingdom's forces headed towards the capital, the citizens were faced with a choice: flee or stay. In the end, many stayed due to them having nowhere else to go. A while later, the Royal Army drafted all the capital's able-bodied citizens into its ranks and set out to confront the Sorcerer Kingdom's forces before they reached the city. However, they were defeated and the entire Royal Army was wiped out.

After learning of the Royal Army's destruction, a defeatist atmosphere took hold of the capital. With the Kingdom's defeat all but certain, those who still could flee the city while the remaining soldiers and Adventurers prepared to make their final stand here. Normal citizens meanwhile, having learned of the Sorcerer Kingdom massacring conquered territories, barricaded themselves indoors; some even preferred suicide over being killed by the Sorcerer Kingdom.

On the Sorcerer Kingdom's side; upon arriving, they surrounded the Royal Capital and set up a base nearby. Following King Regent Zanac's assassination, Sorcerer King Ainz Ooal Gown made up his mind to massacre all the Kingdom's inhabitants that hadn't already surrendered, including the capital's. However, to keep his human sympathizing subordinates appeased, he at least had Eight Fingers evacuate the city's children.

Battle
With Cocytus as their overall commander, the Sorcerer Kingdom's army broke through the city gate and marched into the capital, with the Floor Guardian's [Freezing Aura] killing all the soldiers who tried to block them. For the most part, the "battle" was just that: the Sorcerer Kingdom's forces marched in a straight line to Royal Palace, Ro-Lente Castle, whilst Cocytus' aura froze and killed everyone on their path.

However, the Sorcerer Kingdom's march was interrupted when Brain Unglaus blocked their path and raised his katana to challenge Cocytus. Respecting the enemy's resolve and warrior spirit, the Floor Guardian strikes his Decapitation Fang into the ground and takes out his God Slaying Emperor Blade before assuming a battle stance against him. Cocytus closes the distance between himself and Brain, who is now within his range, and easily ends the warrior's life with one single slash from his weapon. Although the two attacks each other at the same time, Cocytus emerges unscathed from the blow, as Brain's blade was not able to reach him. After swinging the God Slaying Emperor Blade, Cocytus shakes off the blood and fat from the katana and returns it to his Item Box. With the one-on-one fight over, Cocytus pulls his halberd out from the ground and examines Brain's body. Once he takes Brain's katana weapon and commands his Frost Virgins to freeze the warrior's corpse, Cocytus then resumes his walk to Ro-Lente Castle.

Aftermath
After the Royal Capital had fallen, the Sorcerer Kingdom's forces spread out to take the remainder of the Re-Estize Kingdom. While the nobles who did not bother coming to the city's defense intended to just surrender, hoping to be spared in return, Sorcerer King Ainz had already resolved to massacre the Kingdom's remaining population after witnessing their betrayal to their King Regent.

Once the battle was over, the Sorcerer Kingdom had one of its 6th Floor Guardians, Mare Bello Fiore, raze whatever remained of the city to the guild. Afterward, the Sorcerer King had Guren melt away what remained of the city. As a result, the capital was completely erased like it never existed, with only Ro-Lente Castle being spared to serve as a warning to other nations to not antagonize the Sorcerer Kingdom.