Thrymheim

Þrymheimr (スリュムヘイム), informally called the Inverted Ice Pyramid (氷の逆ピラミッド), is an aerial dungeon located above the Great Void in Jötunheimr. This dungeon is the place where the real Excaliber was located.

Description
Þrymheimr is a large inverted pyramid-shape ice structure that is held about 1000 meters or more above the Great Void by the branches of the World Tree that form a net. The base of the pyramid is about 300 meters on each side and its height is about 300 meters as well. Several luminous points are trapped within the icicle, giving the illusion of a gigantic eye blinking majestically, while, internally, it is divided into 4 sections with a myriad of rooms and passages being dug in the ice. Due to the shape of the dungeon, the lower floors of the dungeon are smaller than the top ones, with the 3rd floor being around 70% of the 2nd and the 4th consisting of mostly the boss room. The rooms within the castle are riddled with various complicated passageways, puzzles involving the use of levers, gears or stepping switches to unlock access to the next area.

The entrance to the dungeon is a terrace that protrudes from the top of the pyramid. The dungeon can only be reached with the help of flying beast-type Evil Gods, as none of the fairy races can reach fly to the dungeon by themselves due to their flight being greatly limited or disabled within dungeons.

Setting
Once upon a time, the king of the frost giants in Niflheimr, named Þrym, transformed into a wolf and infiltrated Jötunheimr. He then stole the Holy Sword Excaliber, forged by the blacksmith god Völundr, and threw it into Urðr's Spring at the center of Jötunheimr. The sword severed the thick roots of the World Tree in the spring and, at that moment, Jötunheimr lost the blessings of the World Tree, becoming a desolate world of frost. The water of the Urðr's Spring froze, as it was pulled up by the shrinking roots, thus forming a bottomless pit, the Great Void, where Urðr's Spring once was, while the frozen water became a huge mass of ice, with Excaliber stuck at the bottom-most part of the ice mass, and half pierced into the crust of ALfheim.

Þrym and his subordinates then invaded Jötunheimr from Niflheimr, capturing and imprisoning the previous residents of the world, the Rock Giant Tribe in the various fortresses and castles they had built. Þrym also fashioned the large mass of ice into a linear dungeon-castle, called Þrymheimr, and began ruling over Jötunheimr from his castle, while the previous rulers of the land, Urðr and her two sisters, who no longer had the power they used to have had to retreat to the bottom of a certain spring.

From then on, Þrym attempted to slaughter the Beast-type Evil Gods in Jötunheimr to render Urðr completely powerless, thus allowing the castle to float up to the plane above Jötunheimr, Alfheim, allowing him to invade the land of the fairies. However, after a certain amount of time, due to being unable to easily kill off all the Beast-type Evil Gods, Þrym decided to enlist the help of the fairies, supposedly offering Excaliber as a reward for their help. With this, most of the guardians of Þrymheimr joined the battle in Jötunheimr, leaving the castle much less defended, allowing an opening which Urðr seized to enlist the help of the fairies to defeat Þrym before the Beast-type Evil Gods were slaughtered and to bring back Jötunheimr to its original paradise.

Notable Locations

 * Þrym's throne room
 * The plinth of Excaliber

Fairy Dance Arc
While Tonkii was taking Leafa and Kirito to the center of Jötunheimr, they noticed the large icicle structure from afar. After Tonkii completed its transformation into a winged beast, he took both Leafa and Kirito close to the pyramid, where, using a spell that produced a crystal that could be used to take a close look at distant objects, they noticed the Holy Sword Excaliber being wedged at the very bottom of the pyramid. Even though Tonkii flew near the balcony that lead inside the dungeon, both Kirito and Leafa reluctantly decided to avoid going into the dungeon, as they realised that they could not defeat the guardians of the dungeon by themselves, and instead left Jötunheimr via the roots of the World Tree.

After ALfheim Online was acquired by Ymir and things had calmed down in the game after the new major updates like New Aincrad and Sword Skills, Leafa, Asuna and Kirito with Yui attempted to do a scouting raid on the dungeon, but they were quickly stopped by the abundance of four-armed humanoid Evil Gods and boss-type monsters. Although they all swore to try conquering the dungeon again, due to being occupied with conquering New Aincrad to reach the 22nd Floor, while no one else managed to find out how to reach the pyramid, the dungeon was left alone until December 28, 2025.

Caliber
After finding out that some players had found out the location of the Holy Sword Excaliber, though did not yet manage to acquire it, Kazuto and Suguha decided to call their friends to form a 7-member party to again attempt to clear the dungeon. That day, using the shortcut to enter Jötunheimr via the World Tree roots, they arrived at the canopy of within about 5 minutes and called Tonkii to give them a ride to the dungeon.

After being approached by Urðr and discovering and that if the Beast-type Evil Gods were wiped out, Alfheim would be invaded by the Frost Giants, while rolling back the server would likely not be restore changed fields and the GM support from Ymir was unavailable at the time, the party decided to invade the castle before it could begin invading Alfheim.

After several battles with the floor bosses, several sub-bosses and easily navigating the complex passageways with Yui's assistance, the party arrived at the passageway leading to Þrym's room, they noticed a cage made of elongated icicles on the wall's edge with a female NPC within it. Although almost all of them believed that this was a trap and did not want to take any unnecessary risks, Klein still decided to go against the opinion of the group and freed the character. Although she was offered to leave the castle, she instead asked to join their party in order to reclaim a lost treasure, surprising the party. Despite still doubting whether this was actually a trap, Kirito agreed to let the NPC join their party and, from the new party member bar below his HP bar, he found out that the NPC was named Freyja.

After entering the boss room and a short conversation with the boss, the battle soon commenced. After several minutes of battle, when Þrym's first HP was depleted and his wide-range attack put most of the party in the red and only about 15 minutes remained till Þrym would become victorious, Freyja suddenly asked for their help in finding her lost treasure, a hammer, to return her to full power. After the hammer was found and returned to Freyja, she suddenly grew in size and turned from a beautiful woman to a giant, muscular, middle-aged man, Thor who then assaulted Þrym with all his power, providing an opening for the fairies to use their Sword Skills at full power and Þrym was soon defeated. After rewarding the fairies for their help, Thor took his leave in a flash of lightning.

However, when the castle started shaking, they realized that the quest was not over yet. As the true mastermind behind the plot was Þjazi, the quest would not be completed until Excaliber was removed from its plinth. By using the stairs located behind the throne, they arrived at the lowest part of the pyramid where Excaliber was located. With enough effort put in, Kirito successfully pulled out the sword. At that moment, the roots of the World Tree started re-growing at a tremendous rate and started breaking the castle apart while the bottommost part of the castle broke off from the rest of the castle. With the help of Tonkii, the fairies managed to escape the castle before they crashed into the ground, while the remains of the castle filled in the Great Void, reforming Urðr's Spring.

Known Monsters

 * Frost Giants (Humanoid-type Evil Gods)

Known Bosses

 * Cyclops (1st floor's main boss)
 * Black and Gold Minotaurs (2nd floor's main bosses)
 * Centipede-like giant (3rd floor)
 * Þrym

Trivia

 * The actual castle of Þrymheimr was actually ruled by Þjazi.