The "sky" in the Edge of Reality.
The Edge of Reality is a location within the Shadowlands that seems to have a connection to the void. During Ner'zhul's last stand following  Darkness Falls, he retreats to the Edge of Reality and is killed there.
There are also portals to the Edge of Reality (also labeled 'Edge of Reality' to denote where they lead) that rarely spawn across Draenor that can lead to the [Voidtalon of the Dark Star]. These portals are rare, only allow one person through before despawning, and will automatically despawn if someone who already has the voidtalon tries to enter.