Access is unlocked by completing all of the hardmode primals after the MSQ (quests in the Waking Sands).

The tuning is spot-on thus far (only killed the first boss in the first turn). Not sure exactly how much of a gear check it'll be for the 2nd boss in the first turn and the rest of the Coil. We were very skeptical about Yoshida's comment about less than 100 players clearing all five turns before 2.1, but depending on when 2.1 releases, it might take that long for 100 players to get the tomestones of mythology needed.