Inconspicuous Note (Endless Halls)
This note is found at the end of the puzzle in the Endless Halls. It holds the seventh and final clue for obtaining the 
[Lucid Nightmare], as part of the Riddle of the Lucid Nightmare.
Contents
The way is now open.
To the greatest secret never told.
A fitting end to your journey.
Solution
Since launch back in 2004, the Forgotten Crypt at [39.9, 73.6] just west of Karazhan in the Deadwind Pass has always been closed off to players. It has oft been the subject of conjectures and rumor.
"What secrets are down there?"
"Is there actually a room called 'The Upside-down Sinners'?"
"Did they forget to change the name of various doodads?"
Having gotten this far in the riddle, there's only one way to find out. Head down into the crypts and find The Pit of Criminals. On top of a huge pile of bones at [38.5, 74.6] is the Puzzler's Desire. Open it to the sound of a few fireworks going off to loot the [Lucid Nightmare]!
Patch changes
Patch 7.3.0 (2017-08-29): Added.