Calibur is a side story in the Sword Art Online series, set between the events of Phantom Bullet and Alicization arcs. All based within December 28th 2025, the story is one of several stories published within Volume 08 of the Light Novel series, as well as an addititional Light Noval release titled Calibur SS, which follows similiar events with an alternative ending. Calibur was also adapted into it's own manga and anime series, with publication of the manga receiving 10 chapters throughout the years 2014 and 2015, and the anime being aired as episodes 15 through 17 in the show's second season.

Story

The story opens on the morning of December 28th 2025, only a few days before the end of the year. Kazuto (Kirito) was enjoying breakfast with his cousin Suguha (Leafa) who was busy reading the latest VRMMO news on her tablet. To her surprise, and article had been published that a team of players had discovered the existence of Excalibur, Alfiem Online's strongest weapon ever created. She proceeds to show Kirito the article, as they are no stranger to the weapon. It had been a few months since their encounter with it, seeing it as a dungeon reward found high up above the underground world known as Jötunheimr. However, this did not match the article in how the weapon was rewarded, instead players were being tasked in wiping out many of the world's Evil God beast NPCs

Now with the item's existence known and well publicised, Kirito gets to work forming a party of seven members, the maximum amount of players that can ride on the back of Tonky, a flying beast NPC native to the region, and the one that Kirito and Leafa had rescued in the past. So, with the help of Asuna, Klein, Lisbeth, Silica and GGO's Sinon, the team dive into Alfiem Online, meeting in Lisbeth's weapon shop.

After preparing for what they assume would be quite a vigorous fight, they take a secret pathway leading deep into Jötunheimr. They would find themselves exiting high up above the ground-floor, overlooking most of the region, but still nowhere even close to the dungeon's entry-point. With flight being outright restricted in the world, Leafa summoned the beast NPC Tonky for assistance as it was the only beast in the world that had the ability of flight. With all members of the party climbing onto it's back, Tonky flew in the direction of the dungeon, but instead flying much lower toward the ground, displaying many of the acts of violence many of the other Player Characters working alongside beast NPCs to wipe out all the Evil Gods beast NPCs.

With the party now fully aware of how different their quest is to what the other players are doing, it is at that point in time when they are visited by The Queen of the Lake NPC named Urd. Yui noted that her systems were notably different to most NPCs in the game, claiming that she had the ability of holding a real conversation with the players of the game through the Cardinal System. Urd would inform the party entirely of the situation at hand, claiming that the King of the Giants Thrym was responsible for all the recent actions that had been set in place as a way to rid his enemies. It was he who was also responsible for casting Jötunheimr into the frost as a result of him casting Excalibur into the Urd's lake in the centre of the world, and cutting deep into the root of the world tree. As a result of this action, Jötunheimr was no longer able to receive the blessing of Yggdrasil, which did not only result in the appearance of snow, but the tree also rose up into the fairy world, with the lake, now entirely frozen as a big block of ice, being taken alongside it. Thrym would in the future carve out this lake into his own fortress overlooking the entire world. Urd reminded the party that if they fail to take down Thrym before all of the Evil God beasts are slaughtered, she would no longer have any power, and Thrym would have the complete freedom to destroy the world of ALfheim, an event that no Game Master or System Administrator would be able to prevent.

Arriving at the entrace to the dungeon, the team rush in to complete the dungeon's four floors, each putting up a significant fight. Before reaching the final floor however, the party encounters an NPC known as Freyja. One notable difference that the party noticed is the existence of an HP bar on her character, as they are typically absent on the vast majority of NPCs in the game. While the party feels that her existance is a booby trap set to hinder the party, Klein puts his passion for cute girls in-front of anything other risks and rescues her anyway, and the team forfeit into allowing Freyja to join the party as a Mage-specced fighter.

The party arrive at the fourth and final floor of the dungeon, with the final boss Thyrm awaiting them. The team put try their best to put up a considerable fight, but with his massive HP pool and his attack patterns increasing in difficulty, they were under some serious pressure for time to beat the dungeon before Urd's power would run out. Freyja would then take the time to inform Kirito and Leafa that, if she would to weld her family's heirloom, which had been stolen by the Great King, he power would increase dramatically, to a scale of which would rival the King. With some keen intuition, they would find a heavy Golden Hammer among the treasure, and with some struggle would throw it over to Freyja. As she caught it, her transformation into Thor begun, becoming as tall as the Great King himself. Utilising the mighty power that Thor brought to the fight, the party would swiftly take down Thrym. As a reward for the assistance, Thor rewarded Klein the in-game weapon Lightning Hammer Mjölnir and return back to Asgard.

Pressed for time, the descended further down into the dungeon's secret basement, where Excalibur was resting, cutting deep into the root of the World Tree. With a great amount of effort, Kirito was able to remove the sword from the floor, and as a result, the roots of tree were quick to return to action, reconnecting Jötunheimr into it. With colour and life now returning into the world, the fortress began to collapse, with the platform the party was standing on now falling deep into the pit where the lake once resided. As luck would have it, Tonky reappeared to rescue the party from almost certain death. Unfortunately for Kirito, he had not yet been able to accept the Excalibur into his inventory due to the quest still being in progress, and with it's remarkable weight, he had to sacrifice it to the pit in order to make the leap onto Tonky's back. Sinon however would not accept this fate, utilising a special Retrieve Arrow, she would rescue the falling sword with her remarkable archery skills and return it into Kirito's possession. The party was promptly greeted again by Urd and accompanied by her two sisters Skuld and Verdandi, giving the party a remarkable reward for completing the quest, with Kirito being able to keep Excalibur.

References

• Novel Volume 8 - Fan Wiki