Night Sky Sword

The «Wooden Sword», usually referred to as "The Black One", is Kirito's personal one-handed sword in Underworld, and it is equal to Eugeo's Blue Rose Sword in power. It 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 Volume 10, it's 6 whetstones which were supposed to last 3 years), in the process. It is a Divine Instrument class sword, pitch black in color and the sword's priority class is 46.

Alicization Running
Kirito and Eugeo visit Sadore's shop and he gives them the sword, stating that he won't charge them anything, Kirito practices a few blows with the sword and, to their surprise, he cuts a buckler without even touching it. Kirito practices his Sword Skills with the sword and manages to get to 4-hit Sword Skills, before staining Uolo's clothes and later face 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's arm and is about to be killed by Raios, 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.
 * 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.

Ночное небо