A Demidos of Reality

For this achievement you simply need to kill [npc=84911].

Climb up route: http://gph.is/13g8GbO you can use the rope at the top to help your party members to get up there. The rope exists for 20 sec. Edit: Draenor Pathfinder trivializes this part.

We managed to kill [npc=84911] with a group of 3 in 5 minutes, our ilvl was around 600. The void guy is at 46.0 - 71.7, Socrethar Rise, Shadowmoon Valley. It has 6.4 mil HP (stacked up for 3 ppl) and besides of melee attacks all he does is spamming [spell=170563] a PBAoE, and [spell=170567], both for 10-20k per cast.

Looted some [currency=823]s only but it has a chance to drop a Battle Pet: [item=119431] / [npc=88692].
Also, this guy grants a huge chunk of progress for Assault on Socrethar's Rise. Respawn time yet unknown but seems to be related to the population of the Socrethar's Rise area in some way.

An obvious reference to actress Demi Moore.

edit: As of the latest build, the achievement's name has been changed from Demi More to A Demidos of Reality. :C

Seems like a few good men got offended ;)