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, to convince its Dark Elf population to migrate to the Sorcerer Kingdom; all whilst avoiding the conflict. However, upon learning that the Theocracy would soon be attacking the Elven 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, with Ainz using [Perfect Unknowable] to remain undetected. Arriving at the Elven Palace, they eventually found 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 power, causing Aura and Mare to take a defensive stance while Ainz confirmed that the monarch could not see through his cloaking and began [Buffing] himself. Impressed that the twins could resist him, Decem concluded that they must be his grandchildren.

Having been ordered to not attack unprovoked, Aura started a conversation with Decem to extract information that culminated in the Elf King ordering her to bear his offspring. This caused Ainz to snap and punch 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. Before the twins could answer, however, Ainz falsely claimed that he was their guardian created through their combined power and that the Elf King would need to beat him if he wished to take them.

Amused, Decem accepts the challenge. 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, rolled his eyes at the undead's proposal and summoned Behemot, shocking Ainz since Primal Earth Elementals should be impossible to summon through normal means.

Ainz's Strategy
As Behemot swung its arm at Ainz at Decem's command, the Sorcerer King concludes the Elf King to be a Summoner. Speculating that the Elf King likely has a skill that boosts his summons' stats at the cost of extra MP, the Sorcerer King counters the Elemental's attack with [Wall of Skeleton], which it easily destroys. Using [Mana Essence], Ainz confirms that Decem's MP took a dip following the summon's attack and settles on his strategy for the fight: trick the Elf King into exhausting 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], preventing the Elf King from [Teleporting] away. 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 casts the 10th-tier spell [Mercy of Shorea Robusta], granting him protection against insta-death. In reality, however, this was a boon to Ainz, 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. 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 casts the 8th-tier spell [Aspect of the Elemental] to protect himself against statuses. While Ainz's specialties had been blocked, his primary strategy, making Decem waste his MP, was going splendidly.

Decem Begins Struggling
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 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 can easily beat the invaders with just physical stats. However, recognizing the need to conserve some strength to subdue Aura and Mare, 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 out the remaining two. Ainz then uses a magic scroll to shroud the area in 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 and 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 and asks the Elf King several questions regarding his Primal Earth Elemental. Hoping to extract some information, Decem answers. However, talks break down when Decem demands Ainz lower the [Walls of Skeleton] if he wishes to continue, which the Sorcerer King refused.

Ainz's Counterattack
Almost out of MP, Decem was about to just take the twins and flee when Ainz suddenly lowered the [Walls of Skeleton]. After the Sorcerer King confirms that his aim was indeed to lower his MP, Decem mocks him by stating that he cannot hope to beat him physically. He is silenced, however, when Ainz agrees. Briefly considering that this undead might be a stronger magic caster than him, Decem 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 attack once again. However, having buffed himself beforehand, Ainz easily endures its hits and finishes it off with another [Cacophosus Burst].

Behemot's destruction, along with also having been damaged by Ainz's spells, causes Decem to lose his mind. Ainz 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. Lacking the MP for another attack and noticing the exit behind Ainz, Decem began hysterically running toward it, ignoring the Sorcerer King's order to stop. After [Time Stop] failed to affect the Elf King, Ainz resorts to casting a [Mine] in his path, which explodes as he reached the exit. However, while heavily injured, Decem survives and successfully gets away.

Aftermath
Although the battle ended in Ainz's victory, the Sorcerer King blamed himself for allowing Decem to flee. Aura offers to go after him but Ainz stops her since he would likely be 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, since the Elf king had counter-measures against Aura's summons. Meanwhile, Ainz and Aura would do what they had originally set out to do: looting the treasury.

While managing to flee from Ainz, Decem would not get far. Because, upon reaching his throne room, he was confronted by Antilene Heran Fouche, who had been sent by the Slane Theocracy's Cardinals to assassinate him. In the end, due to having used up his MP fighting Ainz, the Elf King was killed with ease. However, this would cause Antilene to cross paths with Mare, resulting in the two getting into a fight that ended in Mare's victory. Afterward, Mare took her unconscious body to Ainz and he decided to take her with them for interrogation, although he was somewhat upset at leaving Decem's corpse, and his equipment, behind.

Even though he had no intention of taking to the frontlines anyway, Decem's death meant that he could not participate in the Assault on the Elven Capital. As a result, the city fell to the Theocracy's forces.