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The «Night Sky Sword» (夜空の剣, Yozora no Ken?), often called "The Black One", is a Priority 46 Divine Object-class long sword that is Kirito's personal one-handed sword in Underworld. In terms of Priority Class, it outscales Eugeo's Blue Rose Sword.
The Night Sky Sword is made of wood and pitch black in colour. It is less than 120 centimeters long.
The Night Sky Sword was made by a craftsman, named Sadore, from the Gigas Cedar's branch that received the most blessings from Solus and Terraria and which Kirito and Eugeo had taken from the Cedar after cutting it down. It took a year for the craftsman to finish the sword, even breaking three black-brick whetstones from the eastern empire, which were supposed to last ten years[† 1], in the process.
On the 7th day of the 3rd month of the Human Empire Calendar (HEC), Kirito and Eugeo visited Sadore's shop and he gave them the sword, stating that he would not charge them anything. Kirito practiced a few blows with the sword and, to their surprise, he cut a buckler without even touching it. Sadore asked him to pick a name for the sword, but because all of Kirito's previous swords were named by the system, he was not sure what to call it, and so he decided to name it later. Kirito later practiced his Sword Skills with the sword and managed to get to 4-hit Sword Skills, before staining Volo Levantein's clothes and later faced him in a duel using the sword.
Due to the Sword Mastery Academy's rules allowing only Academy provided wooden swords to be used during group practice and exams to ensure fairness, Kirito's sword was only drawn from its scabbard during its weekly maintenance. However, on the 22nd day of the 5th month of the Human Empire Calendar, when Eugeo severed Humbert Zizek's arm and was about to be killed by Raios Antinous, Kirito used the sword to fight against Raios, severing his hands after using «Whirling Current». During the battle, the sword grew several centimeters, allowing Kirito to use it as a two-handed sword instead to use the skill.
When Kirito was arrested by Integrity Knight Alice Synthesis Thirty the next day on the 23rd day of the 5th month, Ronye Arabel, Kirito's valet, attempted to return Kirito's sword to him. Alice allowed this, but confiscated the sword, before returning with both Kirito and Eugeo to the Central Cathedral.
On the 24th day of the 5th month of the Human Empire Calendar, after escaping their prison cells, Kirito and Eugeo were given shelter by Cardinal, who also taught the pair the «Armament Full Control Art». The duo then made their way to the weapons vault on the third floor of the Cathedral, where they retrieved their swords. Kirito then used his sword in the fight against Deusolbert Synthesis Seven, and was able to dispel one of his fiery arrows using the Aincrad-style defensive Sword Skill, «Spinning Shield».
Upon reaching the 50th floor of the Cathedral, Kirito and Eugeo were confronted by the Integrity Knight Deputy Commander, Fanatio Synthesis Two. Kirito engaged her in a one on one duel, clashing his black sword against her Heaven Piercing Sword. Seeing that they were evenly matched, Fanatio activated her weapon's second, recollection, phase, prompting Kirito to activate the first stage of his armament full control art for the first time. The art released a large, black spear, reminiscent of the Gigas Cedar, that shot forward, and collided with a massive beam of light that Fanatio fired. After a brief struggle, the black spear absorbed the light, empowering it enough that it broke through the beam and dealt Fanatio a fatal blow.
Having defeated Fanatio, Kirito and Eugeo advanced to the 80th floor, where Alice awaited them. Seeing the power of her Fragrant Olive Sword's Armament Full Control Art, Kirito challenged her to a duel using only their swords. After that also proved futile, Kirito and Alice pitted the first stage of their Armament Full Control Arts against one another, which resulted in an explosion that blew them through a hole that had been blown in the wall of the Cathedral.
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After falling out of the cathedral, Kirito used his black sword to stab it into the walls of the Central Cathedral to stop his and Alice's fall.
On the 25th day of the 5th month of the Human Empire Calendar, Kirito fought against Eugeo with the sword. When he reached the 100th floor of the Central Cathedral, he used the sword to defeat Chudelkin, and later dual-wielded it alongside the Blue Rose Sword, temporarily turned into the Red Rose Sword, to defeat Quinella. After the battle, Eugeo named the sword the Night Sky Sword when he compared Kirito to the night sky.
The Night Sky Sword rested on Kirito's lap, along with the Blue Rose Sword, during his soulless state.
- The Object ID of the sword is WLSS102382. This ID can be used for certain Sacred Arts rituals, for example, an item locating ritual.
- Kirito speculated that "WLSS" means "Double-Edged, Long Sword, Single-hand," and "102382" represents the order the sword was created in.
- However, in the Web Novel, the sword's Object ID was DI:WSM:1999.
- Although metalworkers and craftsmen in Underworld are supposed to be able to move their crafts with no problems, no matter how outstanding they are, Sadore was only able to lift the sword by one Mel (meter), even after giving his all into lifting it.
- Unlike most swords in Underworld, the Night Sky Sword was chiseled rather than forged. A chiseled sword has a lower priority than a forged sword from the same material would have, which is why chiseling swords is regarded as impractical.
- The blade's length is the same as Kirito's sword in SAO, Elucidator, as Kirito was the one who specified the preferred length of the sword to Sadore.
- The blade and hilt of the sword are able to grow in several centimeters, turning the originally one-handed sword into a two-handed one.
- The sword was named by Eugeo when he compared Kirito to a star in the night sky after the defeat of Quinella.