Duel at Ro-Lente Castle

Duel at Ro-Lente Castle was a duel between Sorcerer King Ainz and Princess Renner's bodyguard, Climb, during the climax of the war between the Sorcerer Kingdom and Re-Estize Kingdom. In reality, however, it was merely charade set up to allow the princess to maintain her good standing with her bodyguard post-defection.

Background
Prior to the Fall of Ro-Lente Castle, Princess Renner had sent away her bodyguard, Climb, to hide the royal jewels throughout the Royal Capital and prevent the Sorcerer Kingdom from acquiring them. In reality, however, the Princess had been a traitor as early as the Demonic Disturbance and Climb was placing the jewels in pre-determined locations to be retrieved later. Sending him away also had another purpose: so he wouldn't witness her assassinating her father as a display of loyalty towards Sorcerer King Ainz, allowing her to maintain her "Golden Princess" persona in front of him.

In the end, everything went as Renner wished: she successfully proved herself to the Sorcerer King and his close advisors. Climb returned to the castle shortly afterward to find everyone dead, with the Sorcerer King on the throne while Renner kneeled before him with her father's corpse next to her. Though Climb was initially confused, Demiurge then used his [Command Mantra] on him to force him to kneel before Ainz, causing him to conclude that he must've forced Renner to kill her father. However, Ainz recognized the boy from his final confrontation with Gazef and ordered Demiurge to stop. Climb then immediately took a defensive position in front of Renner.

In reality, this was also part of Renner's plan: to kill Climb and later resurrect him in her care. However, Ainz decided to go off-script and offered the boy the opportunity to duel him with his life and ownership of the royal sword, Razor Edge, on the line. He even allowed him to wield the weapon against him. Blinded by anger, Climb accepted.

Battle
Climb threw Razor Edge right at the undead sovereign who lost his balance avoiding the weapon. Climb then followed up with a strike from his fist. The blows were hardly noticed by Ainz, who merely grabbed Climb from his breastplate and held him in the air like a toy, and threw him to the ground. The undead told the defeated warrior that the world he existed in was full of inequalities and should accept his limits and die as it is the only true equalizer for all. Climb refused to give in to the despair, and looked at Ainz with defiant eyes which impressed Ainz reminding him of Gazef's eyes when he killed him.

It then did Ainz acknowledge Climb and decided to properly duel him in a test of swordsmanship, conjuring a blade to face Climb and Razor Edge. Before dueling Climb bid Renner to escape and charged at the Sorcerer King. Despite his tries, his blade was parried by Ainz, making no move to attack but rather toy with the warrior. Angered at being looked down upon, Climb focused one strike at Ainz, betting on everything on that blow. His target was the red orb in Ainz's chest believing it to be a weak spot. But the attack was caught dead in its tracks by the Sorcerer King with a single hand. Climb though had been injured by a slash from Ainz's blade forcing him to retreat.

Deeming the duel to be over, as his opponent could barely move with his wound, and eager to claim and study the sword, Ainz then cast [Grasp Heart], killing the boy instantly.

Aftermath
Overall, the duel went exactly as Renner wished: Climb was killed and later brought back to life with resurrection magic. This was done as another sign of loyalty towards the Sorcerer Kingdom: by giving them "her weakness" they had a guaranteed way of controlling her. Being resurrected also left Climb in a weakened state, allowing the former Princess to nurse him back to health.

However, not all went as Renner envisioned. Because Sorcerer King Ainz opted to duel Climb rather than just kill him, she assumed that she had entered the monarch's radar. Unfortunately, having the Sorcerer King to take part in her charade had severely strained Renner's relationship with Albedo. While the former princess initially planned to hold back her true abilities post-defection, this compelled her to prove to the monarch that recruiting her was worth the hassle.

Upon waking following his resurrection, Climb learned that he and Renner were now hostages of the Sorcerer Kingdom and that she had become an imp. Believing the Sorcerer Kingdom forced her into a racial change, Climb continued to be loyal to her and didn't even mind the idea of becoming a demon as well and spending the rest of eternity with her.

After the battle, Ainz claimed possession of Razor Edge, a sword he had long since been interested in. The Sorcerer Kingdom also made sure to recover the Re-Estize crown jewels Renner had Climb hide throughout the Royal Capital due to them being hidden in the predetermined locations set up by the Sorcerer Kingdom and Princes Renner.