Night Sky Sword

The «Night Sky Sword» (夜空の剣), often called "The Black One", is a Priority 46 Divine Instrument-class long sword that is Kirito's personal one-handed sword in Underworld. It is made of wood, pitch black in colour and is more or less equal to Eugeo's Blue Rose Sword in power.

Background
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 3 whetstones, which were supposed to last 10 years, in the process.

Alicization Running
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 Uolo Levanteinn's clothes and later faced him in a duel using the sword.

Alicization Turning
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 is only drawn from its scabbard during its weekly maintenance. However, when Eugeo cuts off Humbert Zizek's arm and is about to be killed by Raios Antinous, Kirito uses this sword to battle Raios and cuts off his hands after using Whirl Current. During the battle, the sword grows several centimeters, allowing Kirito to use it as a two-handed sword instead to use the skill.

Trivia

 * The Object ID of the sword is WLSS102382. This ID can be used for certain Sacred Arts rituals, for example, an item locating ritual.
 * Additionally, the first 4 characters of the Object ID are letters, "WLSS", and is speculated to mean "Double-Edged, Long Sword, Single-hand," and the number, 102382, represents the order this 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 1 Mel (meter), even after giving all he got into lifting it.
 * Unlike most swords in Underworld, this sword wasn't forged, but chiseled. 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.

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