Access to this encounter requires you to collect each of the Mortal fragments (Hope, Grief, Anger, & Ignorance) which can be gotten from card sets or as drops from Atziri in the Apex of Sacrifice. Once you have all 4, you can put them in your map device in a fashion similar to the Sacrifice or Shaper fragments to open passage to the Alluring Abyss.
The Alluring Abyss has the same type of layout and encounters as the Apex of Sacrifice. However, the monster level is higher and the bosses are significantly tougher. Typically, you need a character capable of handling at least Shaper and his Guardians to defeat Uber Atziri. The Atziri Trio, the second boss encounter before Atziri herself, is oftentimes the most challenging aspect of the run. It is suggested that you try to kill off the Archer first as the arrow rain can kill you easily and gets more powerful if you kill the other members of the Trio first.