The Underground Vilefiend Spire

Underground Vilefiend Spire is a dungeon similar to the Vilefiend Spire.

Registrations take place at the same times and use the same entrances as Vilefiend Spire, with one exception...
You will need a key: Mirror Space Essence obtainable by exchanging x3 Mirror Space Pieces with Biyat (Kaslow X:36, Y:265).
This key can be obtained in the classic Vilefiend Spire and grants you access to this content for one week.
Only one key per person per week is required.

Underground Vilefiend Spire currently has 5 floors, and its bosses visually resemble those of Vilefiend Spire.
However, they are much more challenging, with specific mechanics and a focus on elemental damage.