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After the Sorcerer Kingdom declared war on them, the Re-Estize Kingdom began preparations. However, this only applied to territories that were likely to be within the armies' path. Other regions, meanwhile, didn't even bother to evacuate civilians or build fortifications.
However, unbeknownst to the Kingdom, the date the Sorcerer Kingdom had given for the start of their invasion was an elaborate ruse. While the Kingdom prepared, the Sorcerer Kingdom secretly marched their armies over the border and invaded those territories that thought themselves outside the line of fire. Like this, the Sorcerer Kingdom conquered the Kingdom's western and northern regions, cutting the nation off from its allies and isolating the Royal Capital. The Sorcerer Kingdom also halted the flow of information out of these regions, causing the Kingdom's leadership to have no idea that war had broken out.
To complete their encirclement, the Sorcerer Kingdom targeted the port city of E-Naüru. Before this, however, news of the invasion had finally leaked out, when a noble escaped his territory just before the Sorcerer Kingdom attacked and made it to the port. Now knowing they were coming, the city began last-minute preparations. The city's overseer, Count Naüa, called on multiple organizations for aid. A special Magic Caster Unit was formed and the city's strongest Adventurer group, Four Armaments, was put in charge of the defenses. An evacuation order was also given, though most citizens had nowhere to go and thus choose to stay.
On the Sorcerer Kingdom's side; before the battle, Sorcerer King Ainz was confronted by Nazarick's human sympathizers, who pleaded with him to stop the slaughter of innocent civilians. Wishing to see how his generals would react to failure, the monarch chooses to appease the sympathizers by only sending an army of low-ranking Undead to give the citizens of E-Naüru a fighting chance.
E-Naüru's defenders saw that the Sorcerer Kingdom's forces were mostly low-ranking Undead. Two stuck out though: a Death Knight and Death Warrior. Since both of them were close-quarter combatants, Four Armaments decided that the best strategy was a long-range battle. However, arrows were unlikely to do much to the Death Knight and Warrior. This meant they would eventually have to engage them in close-quarters, in which case, the amount of damage they could inflict before they penetrated the city would decide the battle.
Once the undead were within range, archers began to loosen their arrows on them and actually managed to take out a fair bit. After just three volleys, however, the Death Knight suddenly began rushing towards the city gate. In response, the leader of Four Armaments, Scama Elbero, ordered the magic casters to use Fireball on it. Despite hitting it at point blank range, however, the Death Knight didn't seem to have taken any damage and pressed on. Even though they continued hitting it with Fireballs, the Death Knight didn't even slow down and eventually reached the city gate, where the magic caster couldn't aim at it due to a bad angle.
As the Death Knight began ramming the city gate, Scama ordered the city defenders to direct their attention to it and the Death Warrior. In order to get the Death Knight back into the magic casters' line-of-sight, Scama, Lilynette Piani, and her Four Armaments' magic caster faced the undead. However, not wanting to face such a powerful foe directly, they tried to fight it from a distance using the Lighting spell. Unfortunately, their attacks proved ineffective and the gate eventually gave way.
As the Death Knight entered the city, Scama prepared to face it in direct combat. However, despite her teammates and the city magic caster supporting her with buffs and spells, she quickly found herself on the defensive. Still, she continued fighting the Death Knight lest it might swift it's attention to the city, which would result in mass casualties. Scama attempted to lure the Death Knight to the city walls, where the magic caster would have a better shot at it when wails could be heard that the undead army was closing in.
While the city defenders fought the approaching undead, Four Armaments continued their battle with the Death Knight. However, their attempts to harm and restrain it continued to prove ineffective and it was eventually joined by the arrival of the Death Knight. Seeing the situation as hopeless, Scama ordered her teammates to run. However, just then, the Death Knight's head was pierced by a bullet from the sky.
Looking up the city defenders and remaining undead spotted a flying armored giant with a gun. Quickly concluding it was their ally, Four Armaments moved away as the Death Warrior threw one of its swords at it. However, the armored giant just tanked the damage and then unloaded it's gun on the Death Warrior, killing it.
Four Armaments concluded from the Adamantite metal around the armored giant's neck that it must be a member of Red Drop. The floating armor then signaled the Adventurer Group goodbye and flew away. It was unknown what became of the remaining undead. However, with the defeat of the Death Knight and Warrior, they were presumably defeated soon after.
The citizens of E-Naüru hoped Red Drop would continue to participate in battles henceforth and that the Sorcerer Kingdom's invasion would end soon. However, they were very wrong; in fact, this would end up being the Kingdom's only major victory in the entirety of the conflict.
The Sorcerer Kingdom learned of the loss almost immediately, due to Ainz Ooal Gown sensing the destruction of the Death Knight and Warrior. Fortunately, the Sorcerer King did not face any consequences for the loss, as the Floor Guardians believed he was trying to draw out Red Drop. While they suggested launching another attack, Ainz decided that enough territory had been conquered and to divert focus to capturing the Re-Estize Kingdom's capital. The Sorcerer King also hoped E-Naüru's citizens would use this time to flee, and thus keep Nazarick's human sympathizers happy.
On the Re-Estize Kingdom's side; following the battle, they finally learned of the Sorcerer Kingdom's Covert Invasion. Despite the Sorcerer Kingdom's actions though, King Ramposa III remained steadfast in that diplomacy was the Kingdom's best option. However, Crown Prince Zanac disagreed and staged a coup d'état.
Although the Red Drop member, Azuth Aindra, saved E-Naüru, it would actually cost the Kingdom in the long run; as the priorly unknown threat caused the Sorcerer Kingdom to increase security, which would later end up ruining Tsaindorcus Vaision's assassination attempt on the Sorcerer King.