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.
After 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.
To access Tserrina you need to go to the portal room on the first floor after clearing all the other floors in the 2x raid version and click each pedestal to active the portal to reach her. Each cleared floor is indicated by a light beam shining toward the center pedestal.
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