Inconspicuous Note (Ahn'Qiraj)
The third Inconspicuous Note is found in on the table next to the Mind Larva in the Temple of Ahn'Qiraj after beating the Mindcraft minigame by lining up five brains in a row. It holds the next clue for obtaining the 
[Lucid Nightmare]. See Riddle of the Lucid Nightmare for the whole list.
Contents
Deeper than deep.
Awaits your seat.
Solution
In Deepholm, the lowest location to be found is in the Crumbling Depths [58, 26] in the northeastern corner of the zone. Get past where the Colossal Gyreworm is constantly circling to the reach main room. In roughly the center of the room at [63.8, 22.5] is a Dark Fissure object. Interact with it:
- <A fissure in the ground, barely wide enough to squeeze through. It's too dark to see anything, but you can tell that it is very deep.>
<Drop into the fissure.>
A last warning appears:
- WARNING: You are about to fall into a dark fissure. You may not be able to climb back out again. Are you very sure you want to do this?
Hit Accept to enter the Deep Cavern.
In a cavern with no way out is a lonely chair and a Strange Skull. On the back of the chair is a Dingy Plaque, which reads:
<The plaque is old and barely legible. You can only make out fragments of each line of text.>
Supremacy?
Get...
Shirk...
...eke...
The meaning of the words and their layout on the plaque are irrelevant. However, it's possible extract the phrase 'is the key' from the jumbled up letters, leaving the words Racksum Greep. Racksum Greep is the key, or more specifically, a goblin added to Ratchet in Patch 7.3 that just so happened to reuse the NPC ID of The Abominable Greench.
Racksum is wearing a lot of low-level crafted cloth armor, but most importantly, he's wearing the 
[Shadoweave Mask]. Acquire one by whatever means and equip it before returning to the Deep Cavern.
The skull can now be interacted with, so commune with it to reveal the fourth Inconspicuous Note.
Patch changes
Patch 7.3.0 (2017-08-29): Added.