The Unending Dungeon

The  Unending Dungeon or Infinite Labyrinth is a hidden dungeon where Noir learned how to exist and invade with his Great Sage skills. The remarks of the four layers of Ghosts suggest that 200-300 years ago. It is Olivia, not Noir, who first went inside after it was sealed.

In addition to locating the entrance, you need to push the rotating wall on the left to reveal the stairs to a massive iron gate. No matter what kind of magic attack one summoned, it didn't seem like it'd budge in the least. It is also necessary to recite the required spell: "「The Hidden Dungeon only I can enter. Secretly becoming world strongest!」".

Known Floors
1. Floor: Golden Slimes full runner
 * 1) Floor

2. Floor: Several rooms with monsters and traps, one of which is Olivia, whose limbs are chained and crucified.

3. Floor: Open hallway with a Dead Reaper

4. Floor: Hall of the Dead with Ghosts of former Dungeon Seekers who died in the Unending Dungeon

5. Floor: Giant Beasts:


 * Giant Pincer Ants
 * Giant Python
 * Giant Toad
 * Tigerson

6. Floor: Army of Zombies

7. Floor: Forest of Gold full of Rare Monsters and Bugs:
 * Golden Rhinocerus Beetles
 * Silver Stag Beetles
 * Golden Pillbugs
 * Silver Wolves
 * Killer Tree
 * Dory
 * Golden Bee

8. Floor: Maze of Corridors with Ogre Hag and Dark Box

9. Floor: Submerged hall with tons of fish, sharks, and monsters:
 * Bonefish
 * King Octopus

10. Floor: Hallway with Stone Dog and Three Paths: Path of Anger, Path of Sorrow and Path of Laughter.

11. Floor: Volcanic Chamber with monsters:
 * Magmafish

12. Floor: Room full of talking weapons

Trivia

 * This is the place where Noir meets Olivia.
 * The 9. and the 10 Floors were switched in the manga.