Duel of Kings

Duel of Kings was a battle between Sorcerer King Ainz and Elf King Decem that took place whilst the former was raiding the Elf Country's treasury, using the nation's war with the Slane Theocracy as a distraction. It was the Sorcerer Kingdom's first and only major battle in the entirety of the conflict.

Background
Learning of the Elf Country's war with the Slane Theocracy, Sorcerer King Ainz traveled to the country with the 6th Floor Guardians, Aura and Mare, in hopes of convincing its Dark Elf population to migrate to the Sorcerer Kingdom; all whilst avoiding getting involved in the conflict. Unfortunately, the Dark Elves got embroiled in the war when Elf King Decem ordered the mass mobilization of the entire nation after the Theocracy's forces had reached the Elven Capital. As a result, despite previously wishing to not get involved with the conflict, upon learning of the treasures within the capital, the Sorcerer King changed his mind and hatched a scheme to loot the treasury.

While the Assault on the Elven Capital raged on, Ainz, Aura, and Mare infiltrated the city undetected, with Ainz using [Perfect Unknowable] on himself to remain undetected. Arriving at the Elven Palace, they eventually made their way to the treasury. Before long, however, they were confronted by Elf King Decem, who had detected their presence and [Teleported] to their location. Questioning the intruders as to who they were, the Elf King unleashed his overwhelming presence, causing Aura and Mare to take a defensive stance while Ainz confirmed that the monarch could not see through his cloaking. Impressed that the twins could resist his power, Decem concluded aloud that they must be his grandchildren. This statement proved so outrageous that Ainz and the twins briefly considered the possibility of it being true.

Having previously been ordered by Ainz not to start a fight unprovoked, Aura stroke up a conversation with the Elf King to extract information. Whilst being questioned, the Elf King acted exceedingly haughty, which annoyed Ainz. the Sorcerer King finally reached his breaking point, however, when Decem order the two to bear his children and punched the "pedophile" in the face. With Ainz's cloaking undone due to launching an attack, a shocked Decem questioned what an Undead was doing here and turned to Aura and Mare. Before the twins could answer, however, Ainz falsely claimed that he was their guardian created through their combined power and declared that the Elf King would need to beat him if he wished to take them.

Amused, Decem accepts the challenge and Ainz proposes a one-versus-one battle, though he silently [Messaged] Aura and Mare to come to his aid should he get cornered. Decem, however, just rolled his eyes at the undead's proposal and proceeded to summon Behemot to fight for him, shocking Ainz since Primal Earth Elementals should be impossible to summon through normal means.

Battle
As Behemot swung its arm at Ainz at Decem's command, the Sorcerer King concludes the Elf King to be a Summoner, meaning he likely possesses a skill that boosts his summons' stats at the cost of extra MP, and counters the Elemental's attack with [Wall of Skeleton], which it easily destroys. Using [Mana Essence] Ainz confirms that, the Elf King's MP took a dip following the summon's attack and settles on his strategy: tricking the Elf King into using up all his MP.

Ainz feigns shock at his [Wall of Skeleton] being destroyed, causing Decem to gloat at him. The Sorcerer King uses this opportunity to cast [Lopsided Duel], causing the user to move with the opponent when they [Teleport], cutting off all escape routes. Noticing this, Decem questions Ainz as to what he just did, to which he lies that it was a failed insta-death spell. In response, the Elf King cast the 10th-tier spell [Mercy of Shorea Robusta], granting him protection against further insta-death spells, and mocks the Sorcerer King for thinking that he could be weaker than his summon. In reality, however, this served as a boon to Ainz's strategy, since that spell consumes a huge amount of MP.

Decem orders Behemot to attack again and Ainz counters with three more [Walls of Skeleton], two of which are destroyed instantly by the Earth Elemental's punches. The third one falls as well, but not before Ainz further buffs himself using a scroll containing the spell [Piercing Cacophony]. Ainz then puts up three more [Walls of Skeleton] while Decem further protects himself against statuses using the 8th-tier spell [Aspect of the Elemental]. While Ainz's specialties were being blocked more and more, his primary strategy, making Decem waste all of his MP, was going splendidly.

As Ainz repositions himself, Decem mocks him for "running away" and has Behemot hit him in the back, which the Elf King is surprised he survived. Ainz, however, uses the momentum from the blow to finish repositioning himself and puts up several more [Walls of Skeleton]. Getting tired of Ainz using the same spell over and over again, Decem questions his motives, to which the Sorcerer King lies that he is buying time.

Once again having Behemot destroy the [Walls of Skeleton], Decem concludes that Ainz aims to weaken him to allow the Theocracy's forces to finish it off. Although suspicious that his opponent might be trying to get him to use up his MP, the Elf King is confident that his high physical stats would allow him to easily beat the invaders regardless. However, due to his pride as a druid, he does not want to resort to such a "barbaric" way of fighting. With The Elf King needing to conserve some strength to subdue Aura and Mare should they try to resist him, the Elf King decides to start casting offensive magic in conjunction with Behemot's attacks to finish the fight quickly.

With Ainz once again summoning three [Walls of Skeleton], Decem destroys the first one using the 7th-tier spell [Shining Burst] while Behemot takes care of the remaining two. Ainz then takes out a magic scroll and uses the spell within to shroud the area in a veil of mist. In response, Decem uses the 4th-tier spell [Tremor Sense] to feel the vibrations in the ground, only to sense nothing, causing him to realize that the Undead was using [Fly]. Getting annoyed, Decem briefly considered taking the fight outside, so he could finish the Undead and the invaders together, but ultimately opted against it since that would allow him and the twins to escape. Feeling his MP being consumed, the Elf King concludes that he cannot waste any more time and uses the 9th-tier spell [Tempest] to conjure a storm and blow the mist away.

Locating Ainz by using [Tremor Sense] to feel where the raindrops caused by [Tempest] were weakest, Decem dispelled the storm. However, Ainz had used the time to once again create [Walls of Skeleton]. Fed up, the Elf King straight-up called the Sorcerer King a coward. Ainz, however, states that this was strategy. He then asks the Elf King several questions: 1. was he truly going to do nothing about his people dying in the battle? and 2. was Decem's summon truly a Primal Earth Elemental? Hoping to extract some information from the undead, Decem answered: 1. he only cared about the elves that had inherited his strength, believing they might yet awaken during this battle; and 2. the being he commands is called "Behemot", the guardian of the Elf Country. Ainz continued to inquire about Behemot, but Decem stated that he should lower the [Walls of Skeleton] if he wishes to continue, which Ainz refuses, causing talks to break down.

Almost out of MP, Decem decides to just take Aura and Mare and flee. Just then, however, Ainz lowered the [Walls of Skeleton], causing him to stop. After the Sorcerer King finally confirms that his true aim was indeed to lower his MP, which he concludes should be low enough now, the Elf King assumes that he intends to fight him in physical combat. Mocking Ainz by stating that his previous blow on him was merely a fluke, Decem is silenced when the Sorcerer King agrees with him. As the Elf King begins to consider the possibility that this undead might be stronger than him, he slips into denial and orders Behemot to attack. Ainz, however, uses the spell [Cacophosus Burst] to bombard the elemental with soundwaves, which it is weak against, reducing its HP to almost nothing. Panicking, Decem orders Behemot to position itself between him and Ainz before having it once again attack the undead. However, Ainz easily endured its hits and used another [Cacophosus Burst] to finish it off.

Seeing Behemot get destroyed, along with having received a level of damage he never had before from Ainz's [Cacophosus Bursts], Decem begins to lose his mind. Ainz then pretends to offer to let the Elf king escape in exchange for his gear, intending to kill him as soon as he handed it over. However, although humiliated, Decem's remaining pride would not allow him to just surrender. Lacking the MP launch another attack and noticing the stairs behind Ainz toward the throneroom below, Decem began to run toward them hysterically, ignoring the Sorcerer King's order to stop. In response, Ainz tries using [Time Stop] but it has no effect on the Elf King; thus he resorts to casting a [Mine] in Decem's path, which explodes as he reached the stairs. However, while his legs are heavily injured, the Elf King managed to survive and successfully get away.

Aftermath
Although the battle ended in Ainz's victory, the Sorcerer King could not help but blame himself for allowing Decem to flee, believing that he underestimated him. Aura offers to go after him but Ainz stops her since he would then likely end up being left behind, having used up a fair amount of MP himself. Not having time to call for reinforcements from Nazarick, Ainz makes the tough call of sending Mare to finish off the Elf King, deeming him to be the better choice due to the monarch having counter-measures against Aura's summons. Meanwhile, Ainz and Aura would do what they had originally set out to do: search the treasury for anything worth taking.

While Decem managed to flee from Ainz, he would end up not getting far. Because as soon as he reached his throne room he found no other than his child that was taken by the Slane Theocracy, Antilene Heran Fouche, waiting for him on his throne, having been sent by the nation's higher-ups to assassinate him. In the end, the Elf King was unable to put up any resistance against his daughter due to him already having used up almost all of his MP during this duel with Ainz and he is killed by his bastard child with ease. However, this would cause Antilene to cross paths with Mare, who had been sent to do the same thing as her, resulting in the two getting into a battle of their own that ended in Mare's victory. Afterward, Ainz and Aura arrived and, upon being informed of the situation by Mare, took Antilene with them for interrogation. Unfortunately, due to there being a limit to the number of people that could be [Teleported], they were forced to leave Decem's corpse, and his equipment, behind.

Even though he had no intention of taking to the frontlines anyway, Decem being exhausted from his battle with Ainz and later being killed by Antilene meant that he could not participate in the Assault on the Elven Capital. As a result, the city ended up falling to the Theocracy's forces. Ainz presumed that the soldiers also found Decem's corpse and retrieved his gear.