The Void is a common Norrathian reference to a particular dimension or plane populated by the Shadowed Men, Nightbloods, Evil Eyes and Void Beasts.
Travel between the plane upon which Norrath exists and The Void plane is possible through gateways, properly known as "rifts," that can exist as stable or unstable wormhole-like apertures in spacetime. Some of these rifts are recurrent but inconsistent, such as the one in the northwestern region of The Commonlands. Others seem to be quite stable, such as those at the base of the Anchor of Bazzul.
Smaller tears allowing passage between the planes called Void Storms formed throughout Norrath during the initial invasion and more recently during the return of the forces of The Void.
It appears that the current denizens of The Void are diverse and powerful. There appears to be a caste-like hierarchy, with Voidmen being the primary intelligent masters and Void Beasts serving a lesser, soldier-like role. It is unclear if Shadowed Men are an intermediate caste or simply the appearance of a Voidman when appearing incompletely in the Norrathian plane. Evil Eyes also feature heavily in void influenced areas.
It is likely that The Obelisk of Lost Souls (more accurately known as the Tower of Vul) exists in this dimension, as does the Obelisk of Blight.