Pick up either the Fer Orb (red) or the Lek Orb (blue) and proceed through the barrier of the matching color. You can only carry one orb at a time. (For me red should be first or you have to reset this stage)
Note for the Heroic version: there will be 3 orbs to move, and all must be placed for non-orb-carrying group members to cross the barriers.
Kill all the eternal sentries in the side room, then place the orb on the pillar in that room.
Kill the Akeveans that spawn.
The placed orb changes to a Crys Orb (purple). Collect the orb and exit the room.
Place the Crys Orb on the empty pillar in the first room.
Repeat for the other side.
Placing the second Crys Orb back in the main room spawns Nev Dax'Thall between the orbs.
Once the fight begins, pick up a Crys Orb to increase damage done to her. Note: Also increases the damage done to you.
Massive power drains
When she emotes gathers heat to throw your way, she's about to throw Flare of Vex'Fer, an elemental detriment that does 100M+ damage per tick. Cure it quick!
Press the Floor Panel at ( -263, -161, 656 ) Copy/waypoint -262.65, -161.39, 655.94 to open the door to the next room.
Note for the Heroic version: the panel will not be clickable until a group member has gone and stood in the circles of rotating yellow spikes in the side rooms and turned them red. The period of clickability is very brief: one person will need to stand by the panel ready to click as soon as it comes "live" while everyone else runs round switching circles from yellow to red. This applies to all 4 cylinders except each successive cylinder adds another panel that will need to be triggered.
Stand on the small platform at ( -274, -161, 602 ) Copy/waypoint -273.69, -160.71, 602.49 or ( -252, -161, 602 ) Copy/waypoint -252.41, -160.71, 602.48 until it lights up then run into the room beyond.
Kill all the shadelings and shadowbeasts. Note: They cast many arcane and elemental detriments, and have knockbacks.
During the fight Umbral Lord Yzo will casts 'Trail of Darkness' and ports himself into a side room and become damage immune.
Kill all the five a shadowspren in the room he teleports to. To remove his damage immunity kill the shadowsprens, they are non-aggro, each being held by a pair of shissar statues around the edges of the two black mist rings in the side rooms.
While Lord Yzo is immune to damage you will have a life and power drain on you, Shadow Vortex.
Press all Floor Panels near ( -263, -162, 508 ) Copy/waypoint -263.22, -161.69, 507.67.