To unlock the final bonus dungeon in SO4, you need to defeat Gabriel Celeste at the end of the Cave of the Seven Stars. After that, you simply take the elevator in En II down to the Old Road to Sanctuary and speak to Barago. Barago will then take you to the entrance. Monsters here appear as shadowy zombies, so you never know which enemies you'll be facing in combat, and each floor is randomly generated every time you step in. There are no save or restoration spots, and the final boss of this dungeon is 20 floors in.

If you leave the dungeon, the floors are randomly generated again, and you must start over. After defeating the Ethereal Queen on the 20th floor the dungeon goes on for as many floors as you like, simply repeating the bosses of the 19th and 20th floors.