After thwarting the would-be assault on the Sylph-Cait Sith conference, Kirito and Leafa set out for Alne, towards the World Tree, only to be sidetracked by an event and end up in the underworld, «Jötunheimr». With some help from an unlikely ally, they reached the city only to find that the way to the World Tree was locked. In the end, Kirito saved Asuna. Several months later, they, along with other Sword Art Online (SAO) players began attending a special school for SAO survivors, and playing ALfheim Online, but with a new feature added to the game, New Aincrad. Kirito vowed to complete all 100 floors as they all flew toward the castle.
Looking up, there were many lights shining beyond the dim sky.
Those were not stars. They were countless icicles that hung from the vast ceiling and emitted a faint phosphorescence contained inside them. In other words, this was the bottom of a cave, and its size was the problem.
The distance to the faraway towering cliffs was, in real world distance, probably 30 kilometers. The lowest height till the ceiling was 500 meters. A countless number of carved cliffs and ravines, frozen-white lakes and snowy mountains, and fortress and castle-like structures could be seen at its bottom.
With that, the scale of the cave was unbelievable. This underground space, no, it should be called the «Underworld».
Actually, it was just that. This was another field that filled the undergrounds of the fairy world, Alfheim, a dark world of ice and snow, ruled by dreadful evil-god-class monsters. Its name is...«Jötunheimr».
On January 22, 2025, after repelling the Salamander assault on the Sylph-Cait Sith alliance meeting, Kirito and Leafa decided to log out at the first village on the Alne Plateau they encountered, due to having been in FullDive for eight continuous hours. However, the village they landed at was actually a disguised giant earthworm-like monster, which devoured them. After sliding through the earthworm's digestive tract, they were ejected from the other end of the monster's body and landed in a snowbank. When Leafa managed to recover and extricate herself from the snow, she realized that they had fallen to the land of Jötunheimr deep below Alfheim and the World Tree, where the lack of sunlight and moonlight prevented them from flying. Barely avoiding an Evil God that wandered close to their location, they quickly sought shelter at a small stone shrine and lit a fire.
The two spent an hour at the shrine considering their next course of action, until Leafa noticed that Kirito was treading the line of slumber. This would automatically disconnect the user from the their virtual body, exposing their unattended avatars to the danger of attracting monsters. Knowing that this would lead to them having to restart their journey from Swilvane, Leafa roused him with a strike to his head. When Kirito asked her for information on the dungeon to create a plan of escape, Leafa explained that there were four staircase dungeons connecting Alne to Jötunheimr at the cardinal points of the compass, but they were heavily guarded by extremely powerful Evil God monsters, against whom even General Eugene was unable to prevail the first time he entered the dungeon.
Hoping to join one of the occasional parties of Evil-God hunting players and return to the surface world with them, Kirito asked Yui to search for such groups within range, but her efforts were fruitless. Just as Leafa and Kirito were about to leave the shrine in an attempt to reach the stairs, they heard two Evil-God-class monsters fighting each other. Having reached the scene of the battle, they witnessed that the fight was taking place between a three-headed giant and an elephant-headed jellyfish-like Evil Gods, the former of whom was at an advantage. Feeling pity, Leafa expressed a desire to help the jellyfish-like monster, which was being bullied by the giant, to Kirito's surprise. After a moment of pondering, Kirito thought of a plan. He gained the attention of the giant and had it lured to a frozen lake, where the large monster broke the ice layer on top with its weight. Although the giant did not sink completely, the recovered jellyfish dived into the lake and, having the advantage in the water, defeated the giant.
After the tiring event, Kirito and Leafa asked each other about their next course of action, when the «Beast-type» monster suddenly reappeared before them, but not out of hostility. Yui claimed that it did not mean any harm to them. After realizing that it wanted to carry them on its shoulders, the «Beast-type» monster gently picked them up with its long nose and place them on its back. Upon approaching a large void, the «Beast-type» monster stopped moving. Surmising that the monster might be at the end of its journey, Kirito and Leafa decided to find the exit to the upper world, only to be suddenly interrupted by a group of Undine players hunting in the dungeon. After being forced aside so the group could kill the Evil-God-class monster, Leafa again felt pity for the creature as it had attempted to help them find a way out of Jötunheimr. Kirito then affirmed Leafa's feelings and decided to help the monster by attacking the hunting group. One by one, first targeting the mages in the back row, Kirito and Leafa managed to defeat five players before being overwhelmed by the hunting party, until suddenly the «Beast-type» monster let out a huge shout and it began to light up. Soon the water-based «Beast-type» monster sprouted wings and let out a high-class spell, Dispel Field.
Having overwhelmed the hunting group, the Beast-type monster returned to where Kirito and Leafa were and offered another ride, this time flying around Jötunheimr. Its flight path circled them around an inverted pyramid hanging from the ceiling of Jötunheimr, just below the World Tree roots. Kirito and Leafa then noticed a shining object. Using a spell to get a better view of it, Leafa confirmed that it was the Legendary weapon know as «Holy Sword Excalibur». But since they figured that it would be impossible for the two of them to obtain it at the moment, they decided to keep its location a secret and come back for it another time. After leading them to spiral staircase, they bid farewell to their new friend, who they had named «Tonky», and proceeded up the staircase to Alne.
Using the passcode that she had witness Oberon typing, Asuna unlocked the door and attempted to escape from her imprisonment by Oberon. Upon exiting the golden cage and traveling down a path in the World Tree branches, she found herself in a secret laboratory where Sugou Nobuyuki and his subordinates performed their illegal experiments on the trapped minds of the three hundred players they had captured during the collapse of Sword Art Online.
Despite being careful and quiet as to avoid being noticed, she was caught right before she could escape through the system console in front of her. She struggled against her captors, but was soon brought back to the golden cage and placed under tighter security. She was, however, able to steal a silver key card from one of the subordinates. Since the passcode had been changed, she hid the card under her pillow and fell asleep due to fatigue.
Back in the real world, Kirigaya Kazuto and Suguha spent the morning improving their relationship as siblings. Later, they both traveled to the hospital to meet the sleeping Yuuki Asuna. As Suguha watched her cousin Kazuto, she finally realized how she felt about him. After returning home, Suguha activated her AmuSphere and resumed her online life as the Sylph warrior Leafa.
Finding herself at an inn in Alfheim's capital city of «Alne», Leafa met with the Spriggan Kirito and they made their way to the World Tree. When they arrived in the central district, Yui told Kirito that she felt Asuna's presence, and, in his excitement, Kirito flew in the indicated direction in an attempt to meet with her. As the system would not allow them to pass a certain height, they were forced back. However, Asuna felt the voice of Yui and tossed them the silver key card she had stolen. After obtaining the key card, Kirito parted ways with Leafa and left with Yui for the gate that led into the World Tree.
Accepting the challenge of the guardians who protected the World Tree, Kirito fought his way towards the goal. With the odds against him, he was defeated and turned into a Remain Light, though was soon rescued and revived by Leafa. Ignoring Leafa's warnings, Kirito was still persistent in continuing the impossible challenge and suddenly uttered Asuna’s name, leading to Leafa discovering that Kirito was Kirigaya Kazuto and Kirito discovering that Leafa was Kirigaya Suguha.
After learning the truth, Suguha was heartbroken. She logged out of the game and isolated herself in her room. When Kazuto tried to confront her, she finally told him everything about how she felt towards him. She then closed the door between them and sat by herself. After contemplating on why he distanced himself from his adoptive family and how his time in Aincrad inspired him to reconcile with them, Kazuto told Suguha that he would be waiting for her in the game.
Suguha returned to Alfheim, and while considering whether to meet her cousin, she encountered her old companion Recon, the avatar of her classmate Nagata Shinichi. He helped Suguha make a decision, and thus she left to meet Kirito. Upon their meeting, Leafa asked Kirito for another duel, which ended in a tie as both dropped their weapons at the last moment. In the end, Kirito apologized to Leafa for the pain he had caused her.
With the assistance of Leafa and Recon, Kirito once again challenged the guardians. During the difficult battle, Recon performed a self-destruction spell. They were later joined by the Sylph Lord Sakuya and a group of Sylph players with ancient class weapons, as well as the Cait Sith Lord Alicia Rue and the Dragoons of the Cait Sith race. With their help, Kirito was able to make it to the top of the dome. Still followed by the guardian knights, Kirito quickly used the silver key card he obtained from Asuna. While holding onto Yui, they were both teleported. After the warp, Yui returned to her young girl form, and Kirito followed her directions to the golden cage, where they finally reunited with Asuna.
However, Oberon interrupted the group's meeting, and used his power to immobilize Kirito and Asuna, while Yui was able to escape. After Oberon pinned Kirito to the ground with his sword and altered decreased the Pain Absorber level for him, he began molesting Asuna. When the situation was looking hopeless, Kirito received unexpected help from the voice of Kayaba Akihiko. After removing his sword that had restrained him, he accessed the system and logged in under Heathcliff's ID, gaining the power of a system administrator. He used this power to revoke Oberon's administrator credentials, and then lowered the Pain Absorber to the minimum. Wielding his black sword, Kirito challenged Oberon to a duel, which he easily won, slowly hacking Oberon into pieces until he disappeared. Afterwards, Kirito finally freed Asuna from the game, and then spoke with a digital image of the former Kayaba Akihiko. Kirito was then given the «World Seed» program before he left.
Kazuto met Suguha in his room when he returned to the real world and gave her his sincere gratitude. He quickly left on his bike into the snowy night to meet Asuna. Upon arriving at the hospital, however, he was ambushed by Sugou Nobuyuki in the parking lot. Using the experience he had acquired from the game, he disarmed Nobuyuki of his knife and considered taking his life, but relented and instead chose to restrain him. After entering the medical institute, Kazuto told the receptionists about Nobuyuki. Afterwards, Kazuto took one of the guest pass cards and reunited with Asuna, reuniting with her for the first time in the real world.
Several months later on May 16, Kazuto and Asuna had lunch outside of their school. Both attended the special school made for the student players who had been trapped in «SAO». However, the player killing «Red Players» were being treated in counselling and were under observation. Kazuto and Asuna talked about Asuna's father Shouzou, Nobuyuki's criminal sentence, the future of VRMMO games, and also about Kayaba Akihiko, whose death became known to the public in March of 2025. Kazuto had met with Koujiro Rinko, the female graduate student who had helped Akihiko, and learned about Akihiko's aim of scanning his brain and storing his memories and thoughts as digital codes on the network.
Watching the couple from afar, Shinozaki Rika (Lisbeth) and Ayano Keiko (Silica), were sitting near the windows on the west side of the cafeteria. When Kazuto told Rika about the «ALO incident» and what had happened to Asuna, she and Keiko formed an alliance with a «One Month Truce» to allow Kazuto and Asuna to spend time together, which Rika regretted agreeing to.
Along with Asuna and Suguha, Kazuto attended a party at the «Dicey Cafe», which had been reserved for the offline meeting of the «Aincrad Capture Commemorative Party», which was planned by Kazuto, Rika and Andrew Gilbert Mills (Agil). Watching the occupied table of Asuna, Rika, Silica, Sasha, Yulier, Suguha and the other female players, Kazuto met up with Tsuboi Ryoutarou (Klein), and Thinker, who was now married to Yulier.
With the help of Rinko, Yui and Andrew, Kazuto had uploaded the «World Seed» program, called «The Seed», to public servers, allowing anyone with a sufficient server to download the free package and design a new VR game world to match their preferences. In addition, «The Seed» program could also be used for education, communication, tourism, and so on.
Afterwards, the second half of the party was held in «ALfheim Online», which was now back and operating under a new company. Deciding that the role of Kirito the Black Swordsman was finished, Kirito started a fresh new version of the «Spriggan» Kirito by resetting his stats. To Leafa's surprise, when her cousin led her to the meeting place of all their friends, there was a new addition to the game: the rebirth of the floating castle «Aincrad». Kirito resolved to clear all one hundred floors this time and asked Leafa for her help, to which she tearfully agreed. As Klein, Agil, Silica, Lisbeth, Thinker, Yulier, Sasha, and Asuna headed toward the reborn Aincrad, followed by Sakuya, Alicia Rue, the Salamander general Eugene, and many other players, Kirito moved his lips as if uttering a name before joining them.
- The Jötunheimr part originally did not exist in the web version of the novel and was added to the light novel when it got published, but was then omitted from all other multimedia adaptions of the arc.
- There were slight changes to Kazuto and Suguha's argument and Kazuto's inner monologue after the argument, as he did not know of actually being Suguha's cousin in the web version of the novel.
- On page 8, although there are clearly nine races on the map, the description below says there are eight races.
- The illustrations of Leafa depict her as having golden hair, while a line in chapter 8 references it as being green.