Shadow Realm

The Shadow Realm (or the realm of shadow(s)) is a plane of existence visited by the arakkoa. It is related to the Void.

It exists on the and thus probably in the original Draenor since it is said that on Outland there is a "world of shadows" where the arakkoa's true leaders dwell.

In Outland
During the invasion of Outland, the Sha'tari Skyguard worked with adventurers of the Alliance and Horde to collect Shadow Dust (Outland) from Skettis arakkoa and create Elixir of Shadows to see hidden arakkoa. Upon drinking it, Sky Commander Adaris found Skettis Assassins at Blackwind Landing in Terokkar Forest.

Adventurers used the elixir to find and kill talonpriests Ishaal, Skizzik, and Zellek in Veil Ala'rak.

On alternate Draenor
The Arakkoa Outcasts are able to travel to the realm of shadow at will, through use of magics, scrying orbs, and special elixirs. However, crossing over often takes quite a toll on the uninitiated, leaving them grappling with their own minds in confusion.

While within the realm of shadow, arakkoa claim to be able to move more quickly than in the normal world, although this may simply refer to traveling unhindered by their adversaries. Outcasts may hide in the Shadow Realm to escape their enemies, including the Adherents of Rukhmar, who appear unable to perceive their presence there. However, especially powerful individuals may attract unwanted attention from the Shadow Realm's inhabitants while within it. The Outcasts sometimes also hide powerful relics and items within the realm of shadow, usually at shrines to Terokk.

The Shadow Realm of the Spires of Arak is filled with fleshbeasts and shades. Huge squid-like creatures float high above the trees above Veil Akraz, while the lower denizens of the realm feast on the dead, feeding on the stray life essence that seeps through from the living world.

Shadow Dust (Draenor) can be collected from the remains of Face-Stealers and Essence Fiends; this is used to create the elixirs that allow crossing over into the Shadow Realm, and is also used in the rites of the talonpriests.

Speculation

 * Since death never goes unnoticed in the Shadow Realm, the Essence Fiends and Face-Stealers may have come from the Shadowlands.
 * It is likely within this realm that Terokk was sealed by the talonpriests.