Hidden Dungeon

The Hidden Dungeon (隠しダンジョン) is a dungeon that is hidden beneath the Black Iron Palace.

Description
The entrance to this dungeon is located outside the Black Iron Palace and, accessed through an underground water supply. The dungeon has a notably high level difficulty, despite being on the 1st Floor. The dungeon's walls are made of black stones and lined with columns. A safe area lies at the end of this dungeon, which is guarded by The Fatal Scythe.

The Safe Area is a completely white room with smooth walls and floor. A black cube, a GM console, is placed in the middle of the room.

When the boss appears, the surrounding area turns into a shade of violet blue, and some of the stone walls are given a new graphic as well.

Notable Locations

 * Safe Area
 * The Fatal Scythe's Boss Corridor

Morning Dew Girl
Kibaou uses this dungeon to trap Thinker when the latter was considering discharging him for getting his men killed by The Gleam Eyes on the 74th Floor. Thinker is trapped for 3 days until Yulier finds Kirito and Asuna to help her. Yui demands that she comes along as well, and the group reluctantly allow the girl to tag along.

Kirito easily slices through the dungeon’s monsters and they soon see the safe area where Thinker is being held. Yulier runs toward the safe area, and Kirito barely manages to knock Yulier aside to prevent her from being hit by The Fatal Scythe. Kirito prompts Yulier to retreat to the safe area with Yui as he confronts the boss. Seeing that the boss has too high of a level to identify, Kirito soon tells Asuna to leave as well. Asuna disagrees and fights alongside Kirito. The Fatal Scythe knocks both players down, taking away about half of their HP along with it.

Yui steps out from the safe area as Thinker and Yulier teleport away. The Fatal Scythe detects Yui and attacks her, but is stopped by her «Immortal Object» status. Yui then uses the GM console’s «Object Eraser» function, in the form of a large sword, and easily defeats the boss. She then turns to her “parents” and tells them that she remembered everything.

In the safe area, Yui explains that she regained her memories after touching the GM console when Yulier first brought her inside. Yui also says that she wishes to be with them forever, but will be shortly deleted from the system as a foreign object. Kirito intervenes and uses the still-active Admin access and his computer skills to retrieve Yui and save her as an in-game item, «Yui’s Heart», as part of his NerveGear’s environment data.

Known Monsters

 * Four Eyed Frog
 * Crayfish-like monster

Known Bosses

 * The Fatal Scythe

Trivia

 * The dungeon was not available in the beta version of Sword Art Online.
 * The dungeon only becomes accessible after a certain condition on the higher floors is fulfilled.