Ainz Ooal Gown vs Riku Aganeia

Ainz Ooal Gown vs Riku Aganeia was a duel between Sorcerer King Ainz's body double, Pandora's Actor, and the Dragon Lord Tsaindorcus Vaision, who was operating under the alias "Riku Aganeia" by controlling a magic armor.

Background
During the war between the Sorcerer Kingdom and the Re-Estize Kingdom, it was decided by Ainz that Pandora's Actor will have his battle at the Re-Estize pretending to be the Sorcerer King Ainz. Pandora's Actor was to serve as bait without having a World Item equipped to protect himself in order to lure out strong foes like the one he was face-to-face with for a very short period of time. In his battle against this unknown high-leveled warrior, Pandora's Actor while under the form of Ainz attempts to strike a conversation with him. It was to the point of hoping to provoke a reaction out of the foe throughout the battle.

At the same time, while battling the said armor, Pandora's Actor would intentionally throw the fight on purpose in order to carefully analyze his opponent's advantages and favors. He was given the order by Ainz to prolong the fight with this warrior as long as he could, with the objective of attaining as much information as he possibly can surrounding the armor's powers and finding out the identity of the individual utilizing it. In other words, this entire operation was to expose the unknown foe’s abilities. Pandora's Actor was instructed by Ainz to not use any special abilities while in his form to strengthen the summoned monsters like the Doom Lord. Furthermore, the main equipment used by Pandora's Actor in Ainz's form were lower-tier backup items that he had been lent from his creator and master himself.

For Platinum Dragon Lord, his battle plan was to trap the Sorcerer King in the [World-Isolating Barrier] so that the latter can't make an escape. This is to ensure that a one-on-one death battle takes place between just the two of them. Without Albedo to assist the Sorcerer King, Platinum Dragon Lord made it his objective to kill Ainz as quickly as possible before the Sorcerer Kingdom's Prime Minister return to his side. For that reason, while the decoy/bait Azuth lures away Albedo for as long as possible, it is Platinum Dragon Lord's role to be the one to slay the Sorcerer King during that existing time frame without the Sorcerer Kingdom's Prime Minister noticing. Azuth must go on persistently fighting and distracting Albedo to the death until Platinum Dragon Lord himself is able to use all the time he can gathered from the Red Drop adventurer's ploy to finish off the Sorcerer King Ainz.

Battle
When Albedo left him alone to chase after the Red Drop adventurer, Pandora's Actor as Ainz and sharing the same thought process as the original was the only one left in the military encampment. The warrior use [World Teleportation] to try and arrive at Ainz's location. Sensing another enemy was fast-approaching from behind him via teleportation, he temporarily impedes the warrior's arrival with [Delay Teleportation]. Ainz used this opportunity to buff himself with [Body of Effulgent Beryl] and prepare trap spells like [Explosive Land Mine] until his foe arrived face-to-face before him. Upon the warrior's arrival to his vicinity, Ainz's trap activated and an explosion went off because of it.

Nonetheless, the mysterious warrior with four floating weapons (A katana, a spear, a hammer, and a greatsword) emerges unscathed from the destruction. Ainz tries to engage in a conversation with the warrior and get intel out of it. Having failed to gauge his opponent’s reaction through an exchange of words, the warrior himself was not willing to give the Sorcerer King much time to speak, as it unabashedly took an offensive battle stance. Once the hammer in the warrior's arsenal floated to where it could easily grab the weapon, Ainz did not leave that piece of intel to go unnoticed by him. He thought this meant that their target was not Albedo, but rather himself. Since the warrior had not bothered to converse with him at all, Ainz deduced that it was not trying to buy time. Ainz believes that the warrior was probably planning to end this fight by killing him before any backup could arrive. Everything up until this point of the battle had occurred well within the margins of Ainz’s plan. Still, there was no telling or predicting what move the warrior will do next as Ainz does battle with it.

Ignoring the words of the Sorcerer King, the warrior moved its hand in a somatic gesture, causing one of its floating weapons, the giant hammer to suddenly shoot forward at Ainz. Since he is unable to avoid such a high-speed strike, Ainz allows it to hit him without making a move. The hammer's bludgeoning damage was completely nullified by Ainz's [Body of Effulgent Beryl] spell upon coming into contact with his body. Ainz mocks the warrior that he had already taken several precautions to deal with his own weaknesses as an undead. Ignoring what he says, the warrior responds by casting [World-Isolating Barrier] to trap the Sorcerer King from escaping the battle against it. As a result, a space-warping shockwave passed by Ainz, extending to take on the form of a 1-kilometer dome which encompasses their one-on-one battle. Though this was an unknown ability, Ainz could still pick up on certain aspects of it. He shortly deduced that this was an ability that expended HP and quite a lot of it. Ainz had seen his opponent’s immediate loss of HP when he had activated that ability through his [Life Essence] spell. When using [Mana Essence], he picked up nothing in contrast, meaning that his foe was of a pure warrior build with no mana at all.

Looking to pardon the unknown assailant's prior ambush, Ainz decided to reintroduce himself to the nameless warrior that he doesn't know the identity of yet. Refusing to stay quiet any longer with the Sorcerer King, the armor finally spoke and reveal its name to be Riku Aganeia. Ainz suspects there were some truths and lies mixed together to it. Recalling how the bards sang tales of the armor that appears supposedly platinum, Ainz narrows the individuals down to be only two out there who most likely have ties to it. Ainz presumes the armor was an entity that was not humanoid or someone by the name of Riku Aganeia from the Thirteen Heroes. He offers the warrior Riku a chance to submit to him and become a subordinate of the Sorcerer King. As the latter did not respond to him, he implores Riku even further to take his offer into consideration, willing to put an end of conquering the Re-Estize Kingdom for the sake of recruiting the warrior into the Sorcerer Kingdom. In the end, the warrior declined his offer and it resumes fighting with him once more. The warrior Riku cast [Cloak of Light] and that led to his armor to begin glowing. For a brief moment, Ainz thought that it was the Sun reflecting off of his armor until he noticed that Riku’s HP had just decreased. Through seeing how Riku activate its ability, Ainz had concrete proof that Riku’s abilities were powered by his own health pool.

Ainz attempted to escape the [World-Isolating Barrier] dome casted by the warrior using [Greater Teleportation] but failed. Seeing that he cannot leave the dome, he was also able to keenly take notice of how the barrier did not stop him from teleporting within its boundary but outside it. Realizing the crucial intel that was and how far away the warrior is from where he is at, he decided to use the opportunity to experiment on the barrier for the meantime until Riku catches up to him. Ainz tosses a gold coin on the wall of the barrier and it bounces off. At the same time, he casts the [Lightning] spell at a pre-calculated angle to penetrate the barrier but it didn't get pass through the wall.

Just as he became satisfied with the results of his experiments, his [Delay Teleportation] spell activated once again when the warrior appears nearby via [World Teleportation]. For the second time, Ainz cast [Body of Effulgent Beryl] and stood there, with his back towards Riku. Once Riku arrives to his destination, he was immediately hit by the same bludgeoning hammer like before. Although the bludgeoning attack was completely nullified by [Body of Effulgent Beryl], he felt somewhat dumbfounded and surprised by how that offensive move was handled as it sent him flying and pressing against the barrier when that was supposed to not happen.

However, Riku’s attack did not follow this rule. Normally when an attack’s damage had been nullified, its secondary effects would not apply. Although he asked the warrior about what he did to the weapon, the latter answers it differently much to his annoyance. Not responding to Ainz's question, he only warns the Sorcerer King that even if he was to try teleporting away from him, the latter will not be able to successfully utilize it in escaping outside this barrier. He threateningly stated that his fate was to be perish here by his own hands. The hammer quickly returned to Riku’s side at the same time it was launched. Along the way, Ainz discovers that the four weapons surrounding Riku are a bit different they were from before as each of it appears to be glowing white. Ainz watches silently as Riku’s health had decreased even further. With all the weapons now shining, Riku appears to have lost more of its health increasingly than when he activated [Cloak of Light] on his armor. Ainz ponders whether this was because he had to apply that ability to each weapon separately or because teleportation had cost him a bit of HP too, leaving more to desire from this fight.

Having seen the warrior's resolve to fight him to the death, Ainz magically pulled out two floating weapons of his own via [Twin Magic: Obsidian Sword] and attempted to clash blades with it. Riku deflected one of Ainz's two swords with his greatsword. To his surprise, he can't help but be baffled by how nonhuman Riku's dodge was in response to his other blade slash. Riku unnaturally dodge it without problem by performing a sideways somersault at head's height. Normally when a humanoid jumped, they would bend their knees and apply force on them. There should be reflexive movements before the actual jump. However, Riku skipped all of that steps, applied no force at all, yet still performed a sideways somersault in the same stance he had this entire time while fighting him. Ainz attempted to land blows on Riku once again but his twin blades were easily deflected by the other weapons floating around the latter.

Under Riku's control, the greatsword, as if it had a mind of its own, flew at tremendous speed towards Ainz. In response, Ainz cast a defensive tier spell, [Wall of Skeleton] to shield himself from the incoming attack. As the newly-created wall was instantly destroyed by the greatsword in one fell swoop, the next thing Riku did was to dish another counterattack utilizing the greatsword to cleave him from top to bottom. In place of where the [Wall of Skeleton] stood, the honed edge of the greatsword pointed towards Ainz as it hovered. Ainz had thought that it would return to Riku’s side after that hit, but instead, it flew towards Ainz again. On the contrary, Riku was standing still in his original position, in his original pose, letting the weapon go in pursuit after him. From his observation of Riku's pose over the course of their battle, Ainz had a feeling that he and his armor function like a puppet but cannot confirm if that was the case. He speculates that this isn't something like [Psychokinesis] controlling the weapons, but the armor too. Ainz starts to harbor doubts whether he was fighting a living person or not, thinking there is the likelihood chance that the armor may be empty inside and controlled by someone.

Seeing how Riku is planning to attack him once more, Ainz pulled out a Blasting Staff to intercept the heavy blow and hold off the formidable warrior. After blocking the blow, the amount of pressure exerted on Ainz made him feel as though he was sinking into the ground. Ainz tries to cast [Grasp Heart], thinking whether it could have immunity to necromancy or resistance against negative status effect. before using [Greater Teleportation] to instantaneously move above it on air to avoid the brunt of his enemy's greatsword strike. After doing so, Ainz cast [Body of Effulgent Beryl] for the third time as a precaution. When Ainz retreated to the sky after taking that heavy below, the warrior follow suit and ascended on air and chase the latter from above once he spotted where it was at. With the two obsidian swords being meaningless in his situation, Ainz dropped the two floating obsidian swords above Riku while descending down from the sky to which the latter easily deflect away.

After descent, he was met with a spear, one of Riku's weapons thrown down at him from above. Upon dodging that attack, he was then confronted by Riku who shortly descended from the sky after him. Seeing Riku change stance by gripping its greatsword, Ainz tried to hastily intercept his attempt to attack the former, but only to be deflected by the floating weapons like the latter's katana. He tries to slow the warrior down with [Call Greater Thunder] yet that attack alone didn't faze or disrupt Riku's movement whatsoever. As he fast-approach Ainz without slowing down, the warrior Riku painfully tanks the undead's powerful lightning spell along the way while continuing onward with his charge towards the Sorcerer King. It was not as though Riku hadn't taken damage, Ainz could see his HP pool falling via [Life Essence]. Now that Riku was within his range and is about to strike him, Ainz swing his staff back and forth towards the warrior who tanks his physical counterattack at the same time. Between the two strikes from both sides, Ainz inflicted a knockback effect on Riku to push him far away while he felt the floating katana’s strike from Riku only grazed him by a little, barely touching his sternum.

So far in this fight, Ainz was able to successfully discern that Riku’s tactic up till this point had been to rely on his auto-attacking weapons in battling him. Though Riku had wielded his greatsword, he had not used any special abilities or martial arts to go along with it. Right now, Ainz did not have a firm grasp on what his abilities are as a warrior at all.

Afterwards, he uses this chance to cast [Summon Undead 10th], conjuring forth a high-tier undead creature called the Doom Lord as replacements for his two floating obsidian swords to assist him in the battle against Riku. As Ainz, Pandora's Actor order the Doom Lord to activate its special ability [Ruinous Night] and be on guard for Riku, further commanding the summoned monster to assess whether or not the armor warrior they're fighting is a living creature. After the Doom Lord activate its ability, its HP was falling faster than ever, the cost for a short term boost to every one of its combat related stats. While so, Riku closed the distance between himself and the Sorcerer King, fearlessly charging forward at Ainz without any hesitation whatsoever. Faced with the rapidly-approaching Riku, the Doom Lord tightened his grip on the War Scythe weapon it had multiples of. Preparing himself to be a spectator while the Doom Lord fight in his stead, Ainz made sure that he would not be targeted by Riku, by pulling away from the two combatants.

Now that it was time for Ainz to understand the true extent of Riku’s power, he lets the Doom Lord uses its scythe to clash with Riku's floating greatsword. As an ear-piercing noise echoed around the two, neither of them backed off a single step, nor were they knocked back. Ainz deduced that they must have had similar levels of strength for that to happen. After that, the Doom Lord's scythe continued to clash against Riku katana at high speeds, their metallic rings droned on. The greatsword’s hack was rendered inert by the scythe and the scythe’s pierce was blocked by the shield-like hammer. The darting spear was deflected by the scythe’s handle as the Doom Lord gracefully dodged the greatsword’s cleave. Soon enough, to take advantage of the distance created by the dodge, Riku leapt forward. While both were on equal footing in terms of offensive and defensive capabilities, Ainz felt Riku had more tricks up his sleeve than the Doom Lord. Ainz heals his Doom Lord with negative energy using [Negative Burst]. Although black ‘rays of light’ began pulsing from Ainz as it swallowed up its surroundings through casting that spell, the warrior Riku did not take damage from the spell at all.

Speculating whether this was because Riku had either immunity to negative energy, a racial trait, job trait or it had something to do with his equipment, Ainz continues spectating the fight. While the sounds of both the Doom Lord and Riku's weapons colliding with each other rang out non-stop, Ainz cast his next spell which was [Perfect Unknowable]. Ainz, now an unknown, appears out of the shadow of his tank, the Doom Lord, to circle around them while under complete invisibility. However, Riku was able to see past the spell, as the katana flew towards Ainz at a speed that he could not dodge. As a result, Ainz was pierced through his robe at the abdomen region by the weapon. Fortunately, Ainz did not take any damage due to his immunity to piercing damage. Nonetheless, Ainz still scuttled behind the Doom Lord’s figure to shield himself once more. At the same time, the katana suspended midair, then began to slash towards the Doom Lord instead rather than him this time around. Riku wanted to target Ainz directly given how the levitating weapons were all pointed at him, but with the Doom Lord’s presence in front of it, they could not reach the Sorcerer King at all.

After rough calculations of his available options given the current status of the battle, he arrived at the conclusion that he should be spamming offensive spells. While contemplating over which strategy to imploy next, Ainz decided to cancel the casting of his Doom Lord's attack spell. Ainz observed the two combatants’ respective attacks and defenses. The Doom Lord was being suppressed ever so slightly by Riku, but neither were seemingly taking significant damage. Ainz learned that the Doom Lord could not gain the upper hand over Riku as none of its skills, negative energy attacks, or spiritual attacks had any effects against it. Analyzing this battle, he believes Riku was of a race which had similar properties as Golems or other Constructs as it was able to talk freely with him. Meanwhile, Ainz gave the Doom Lord an order, making it release more black smog upon receiving his command. Due to this, the Doom Lord's speed and offensive capabilities were raised even further, to the point where Riku’s armor began to take damage.

However, the rapid loss of HP on the Doom Lord’s part caused it to disappear not long after. Ainz had timed for this, as he cast another [10th Tier Summon Undead] which conjure forth a new high-tier undead being, the Elemental Skull to his side. Ainz made it fall back behind him and took its place at the frontline of the battle with Riku. On the other hand, Riku did not raise his guard any further even though a magic caster was now in front of it. While remaining silent, he quickly closed the distance between himself and Ainz and began attacking the said magic caster. Ainz used the experience he had gained through his warrior training with Albedo to block Riku’s slashes. He could only block one slash out of five, it was basically a one-sided pummel. As Ainz's Blasting Staff was ignored, the greatsword, spear, and katana began their attacks simultaneously. For the hammer, after three nullifications from [Body of Effulgent Beryl] using it, Riku had decided to not used the hammer again afterwards.

Having bore witness to the Doom Lord’s battle, Ainz knew he could not serve as an adequate vanguard. Still, Ainz’s struggles at being a vanguard began to see some returns as spells began soaring through the air from his back. At the same time, Ainz cast [Vermillion Nova] to scorch Riku yet the latter showed no signs of slowing down as the greatsword cleaved towards him again. Even though his body was bathed in fire, his swordsmanship remained calm and steady. Ainz had the Elemental Skull cast [Polar Claw] which began clawing its way towards Riku for damage. As Ainz memorized the amount of damage Riku took from the two spells, he received a combined strike from both the spear and the katana. In unison with the Elemental Skull which now cast [Mist of Super Acid], Ainz also used [Call Greater Thunder] along with it to strike Riku together all at once. Riku was instantly surrounded by a mist of strong acid and so were his weapons.

While the weapons around Riku were damaged, Ainz who was well within the AOE range, emerge unscathed due to a special condition applied to the spell. Riku’s HP loss from the acid was notable. Of the four elements surrounding it by [Mist of Super Acid], Riku took the most amount of damage from acid than the other elemental attacks. That said, the actual percentage of HP loss was still low. Lost in thought, Ainz managed to somehow perfectly bat the katana flying towards him away with his staff. The spear was blown away as if the knock-back effect had activated by the staff much to his surprise. Realizing this, he discovers that the floating weapons counted as a part of the wielder’s body after taking into careful consideration the conditions how the Blasting Staff's knock-back effect activated. He deduced that the various weapons floating around Riku had its own respective HP too beside the armor. Sensing that the Elemental Skull was about to cast [Seven Trumpeter], Ainz immediately canceled the cast. Ainz silently cast an [Message] as Riku retreated as if he was chasing after the knock-back katana. The katana then returned to its original position where Riku was at.

As Ainz finished contemplating what the mechanics behind his opponent's armor and weapons are, he asked Riku that this fight between them should be good enough for a stopping point. However, before Ainz got the chance to complete his sentence, Riku quietly slashed towards him as he glided along the ground without intending to speak back. Despite being in the midst of Riku’s attacks, Ainz panically calls for the warrior to stop what he was doing, going so far as to throw his staff behind himself and kneel down before it. Much to the warrior's confusion, Riku halted the attack on Ainz, stopping and suspending the greatsword from fully reaching its way onto the latter's head. Because Ainz was immune to critical strikes, he wasn't too scared to lower his defenseless head. He gave an order to the Elemental Skull at the same time.

When the discussion between Ainz and Riku happen, the former reasons that the warrior had no real reason to fight him. Ainz attempts to reason with Riku that this whole thing started because the Re-Estize Kingdom stole the Sorcerer Kingdom's grain which was meant to be transported over as food aid to the Holy Kingdom. Ainz made the call that the Re-Estize Kingdom was in the wrong of the two nations as it is evil for attacking their grain carriage. However, Riku states that Ainz went too far with this war, believing that there should have been better ways for him to deal with this conflict. In response, Ainz argued that Riku was not the victim of this war and asked how the latter would have dealt with this when the grain that his countrymen toiled to produce was stolen away. Riku thinks otherwise, reasoning that if Ainz did not possess the power that he have, things like this situation between the two would not have turned out the way it is now. Riku proclaims that people like Ainz with power have to be careful about how they use it and take responsibility for their actions, declaring himself as the protector of the New World. Saying all that, he can't help but lament at the deeds both his father did and those who gathered around his mother as wrong, seeing that their absolute power brought corruption to the world and is the source of all faults. Riku admits that although the root of every wrongdoing could be traced back to his kind, he had no intention of seeking forgiveness for that. Hence, refusing to let Ainz continue on with his current path of destruction, he ends their discussion off with a swing from his greatsword at the magic caster.

Feeling guilty for executing the defenseless Ainz, Riku had his greatsword swung at a much slower speed than before. On the other hand, Ainz had order his Elemental Skull, which he had commanded to be on standby earlier before the discussion began, rushed into the greatsword’s trajectory and took the hit. Nonetheless, after hearing Riku's reasoning for his involvement in the war, Ainz exploited and guilt-tripped the warrior to hamper the latter's actions. As a result, Riku's movements were much tardier than before from listening to Ainz's words. Meanwhile, Ainz capitalized on this opportunity to roll backwards as he commands the Elemental Skull to rush in between them and block Riku's greatsword attack. The Elemental Skull cast its spell while Ainz yelled. Riku ignored the skull and charged for Ainz. The Elemental Skull attempted to stop him, but due to its size and a lack of skills to do so, it failed. Hastily, Ainz cast his spell [Wall of Skeleton] to erect a wall to keep both Riku and the Elemental Skull on the other side of him. When the warrior saw this, Riku angrily calls Ainz out for looking pathetic in the state the latter was in while attacking both the Elemental Skull and the wall. The skeletal wall soon crumbled immediately at Riku’s strike. Though the Elemental Skull had cast multiple [Vermillion Nova]s to reduce Riku’s HP, the former was still far from defeating due to the latter's abnormally high magic resistance.

Understanding this much of the warrior's resistance, the next spell Ainz chose to use and cast was the [Temporal Stasis] spell on Riku. However, Ainz found that his spell was not just resisted, but nullified completely, indicating to the former that Riku had time-stop countermeasures. At the same time, Riku had his floating greatsword swung towards Ainz, while he made the hammer swung towards the Elemental Skull. Ainz tanked the greatsword’s damage and as a precaution, cast [Greater Break Item] on the other weapons flying towards him. It was not just resisted, but nullified again, further confirming to Ainz that the various floating weapons indeed counted as Riku’s body as a whole.

As the Elemental Skull took a massive amount of damage, the fight between Ainz and Riku came to an immediate end when Albedo intervenes to save her master. Albedo arrived at the scene of the battle by jumping down from the sky, passing through the [World-Isolating Barrier] barrier without problem due to having a World Item equipped on her. Riku can only looked into the air in panic when the female warrior rapidly descends and lands on him with a powerful halberd strike from her. While Albedo instantly swung down her halberd with the intention to split his head open, Riku raised his greatsword and spear in a cross to block her attack. The impact from the halberd created by Albedo was sizable to the point that both of Riku’s feet were sunk into the ground. Before long, Riku was quickly blown away to the side. Enraged by how Ainz was treated in the battle, Albedo began stomping on Riku's chest much to the latter letting out a painful cry and clanging noise. Shortly thereafter, the distance between the two was closed in an instant as Riku received an attack with enough force by Albedo to send him into orbit. The sound of metal clashing together rang out at an ear-piercing volume. Riku used two of the floating weapons to block Albedo's attack. While so, he flew backwards with all of his strength. Not with a jump, he flew back without his feet touching the ground.

In the end, Ainz told Albedo to stop picking a fight with Riku which she did by halting her attack on the said warrior. Despite that, Albedo stood silently by Ainz’s side, using her own body as a shield against Riku after she stopped her attack. No longer wishing to fight against Albedo, Riku began ascending to pull some distance between them. Ainz once more ask Riku to become his subordinate, even willing to go as far as to offer the warrior everything he have ever wanted for in the hopes of having him join the Sorcerer Kingdom. Riku not looking to accept the deal, stood silent and not answer Ainz's offer. Although Ainz got no reply from Riku, he informs the latter that the Sorcerer Kingdom will always have its doors open to him whenever the latter is ready to come and visit the nation anytime it want. Ainz whispers to Albedo whether Riku is still looking to fight him, to which the latter said he was not. However, since Riku is not retreating or running away from their standoff, Albedo suggests Ainz that it would be best for them to defeat him right here, right now. She reasons that it should not be too difficult for the two of them to easily take down Riku for as long as they both attacked at the same time. However, before they could come to a consensus, Riku had already disappeared via [World Teleportation] from the battle not too long after the two ask about it. The [World-Isolating Barrier] created by Riku also began dissipating shortly thereafter until it is fully gone.

Feeling as though they had successfully completed their reconnaissance mission, Ainz praised Albedo for her hard-work in fulfilling their plan. As Albedo believes there might still might be people out there surveying them, she suggest that it would be wise of them to return back to Nazarick first before they discuss any further about their investigation. Heeding her advice, Ainz agrees and undoes his summoning of the Elemental Skull. Afterward, Ainz casts [Greater Teleportation] to retreat to the Great Tomb of Nazarick with Albedo.

Aftermath
After his battle with Riku, much to his suspicion, Ainz confirmed that the warrior neither used skills nor martial arts the entire time of their fight together. Ainz eventually acknowledge the way on how that the armor is fighting him seem like as though a mere puppet like Riku was being manipulated behind the scene without a wearer. He found the armor's movements made it all the less believable to him that a living being like Riku was in that armor to begin with. In addition to that, based on the analysis of every bit of intel he had so far, Riku must have had defense-focused job classes. According to Ainz (Pandora's Actor), he was estimated to be around level 90.

It was later revealed that the Ainz that was fighting against Riku was not the true Sorcerer King and in truth but a transformed Pandora's Actor who was sent out into the battlefield in order to bait any powerful opponents without being equipped with a World Item in order to lure them out and gather information on any unknown threats to reveal themselves during the war for future preparations. Thanks to this ploy, Nazarick had successfully gained new information on a potential threat to the Sorcerer Kingdom and has potentially tricked Riku into thinking that the Sorcerer King was much weaker than him as an potential advantage in future battles.