A-VOID-ance

As previous posters have already mentioned, the first thing you need to do is to enter the void realm via one of the portals placed on the top of various altars.

Altars are placed both, on the bottom and top level of quest area but I recommend to enter the one on the very top, on the right to the Kenos altar.
While inside the void realm, you will probably want to do One of Us! One of Us! first. Anyway, when you are ready to take on Kenos, head to the big altar with three stones around it. Make sure you are actually inside the inner part due to 2 facts: you will slowly lose your bound to the void realm once outside the bubble, and the more important thing: you can aggro mobs patroling area easily.
Avoid pulling Void Terrors and various Voidwalkers.

As soon as three ppl click on the stones, the monster, Kenos the Unraverel will appear and the fight begins.
He is able to cast various AoE spells, mostly on his main target, so you might want to face him away from your group.

During the fight, additional adds will appear to protect their master. For each of them, boss will recieve buff which reduces damage taken from any sources. In order to get rid of the buff, you have to kill the adds.
Here comes the tricky part. Boss will also cast debuff with status icon matching achievement equivalent which will do AoE damage after expiration. This hits foes i.e voidlings too.

In order to get the achievement, you have to avoid getting close to the adds when debuff is present - it is casted only on one person at a time and purple indicator in placed on its target. The radius is about 25 yards.
Important: don't lose faith if debuff hits the adds, it has to kill them for achievement to fail. Yes, you have read correctly, the AoE doesn't kill the adds immidiately, it will hurt them by about 50%.

Fight steps repeat untill boss is dead.
If you fail the achievement, the summoning ritual won't be available for certain period. I don't know the exact number, but it seems to be about 45min or higher. Try to switch realms via group tool finder and see if stones are clickable there.

Is seems that you do not need shadowmoon campaign or unfinnished daily in order to summon the boss.
Worth notice: there seem to be a bug currently with void realm that prevents you from leaving it even if you are not inside the buddle - probable reason: staying in the bubble for longer period of time.

A pun so good they had to make it twice.

Shadowmoon Enclave in Spires of Arak:

Assault on Pillars of Fate or Assault on Pillars of Fate

There are several void portals, but, i think, (72.27 23.62) is the closest to rares. Here is portal location video: http://youtu.be/ceqJilhT-Bw

Achievements

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• A-VOID-ance | http://youtu.be/jjoI17bpw5A
  Enter void portal. Need 3 ppl to summon rare. Keep away from adds with debuff [spell=171057].
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• One of Us! One of Us! | http://youtu.be/j6ice2whScw
  Enter void portal. Kill mobs.
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• Would You Like a Pamphlet? | http://youtu.be/H30GFP8SNeg
  Collect Forbidden Tomes
  
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  • /way 74.62 31.22
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  • /way 74.27 27.73
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  • /way 74.6 31.2
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  • /way 72.76 32.50
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  • /way 71.7 27.9
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  • /way 70.51 24.15
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  • /way 69.73 20.0
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Rares

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• [npc=85037] (70.6, 24.2) Void World http://youtu.be/jjoI17bpw5A | 3 ppl to summon
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• [npc=85036] (72.6, 19.6) Void World http://youtu.be/HfZOSttuWPQ
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• [npc=85026] (72.6, 19.6) http://youtu.be/IicPuxflHng Can drop [item=119178]
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• [npc=85078] (74.0, 32.4) http://youtu.be/0nLj3EPO5Vc
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• ... probably there can be more, but i can't find them on wowhead.
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Will give [faction=1718] reputation for Shadowmoon Invasion! Garrison Invasions