In Vex Thal: Aten Ha Ra is the end-game boss of the Shadows of Luclin expansion. Her Spawn time is 7 days or instant after a server-down and her total hitpoints are around 1.75 million.. "Akevan Warders" present in her room indicate other mobs in the zone are still up and that she will be casting a 100k Deathtouch often if engaged.
In the Plane of Shadow: Located in the palace entrance room, just north of the zone-in area. For all things Rain of Fear, check out this overview of the expansion.
Immune to stuns.