Access is gained by the quest Finding the Dain when all mirrors are collected and placed on the first floor and a group version of Tserrina is defeated.
This is the x2 version of all 3 Heroic zones. It contains all 7 floors as well as Tserrina.
Clearing this zone up to Tserrina is needed to progress in Normal Mode x4 raid end encounters and all Challenge Mode x4 raid encounters because of a archetype specific debuff that is granted. The fighter debuff is also needed to kill Tserrina.
The information below has been verified with in-game data and the information on this page is correct, even if it looks stupid or makes no sense.Fact: zonename is Tower of Frozen Shadow