Voidwalkers are demons created from the chaos of the Twisting Nether. Their touch is misery, and it can momentarily blind a creature's awareness to other threats. These hulking blue creatures look vaguely humanoid. It seems to appear from within a black cloud as dark as the void. A voidwalker’s sole purpose is to unconditionally serve a master. Thus, travelers rarely encounter them alone. The voidwalker is often a warlock’s choice fel companion for its ability to intercept a single foe and keep him or her occupied. Many warlocks also prefer to do without the mischievousness of an imp or the jealousy of a succubus, preferring instead the unwavering, silent loyalty of a voidwalker. Voidwalkers understand but do not speak Eredun. A voidwalker does not hesitate to follow even the most suicidal commands its master gives. A voidwalker’s master often sends it to block a threat, leaving the master free to cast spells or use other abilities without interference. A voidwalker takes its orders literally; when out of communication with its master, it continues to obey its last command, even if that command is tactically unsound. The touch of a voidwalker brings a victim’s most painful memories rushing to the surface; for a brief moment, she can think of nothing but destroying the source of this mental anguish. Like elementals, voidwalkers, whether summoned to Azeroth or arriving of their own free will, wear a set of magic bracers. These bracers form automatically when the voidwalker leaves the Twisting Nether, tying the voidwalker to Azeroth and allowing it to exist outside of the Twisting Nether. A voidwalker that loses its bracers also loses much of its power and might inadvertently return to the Twisting Nether. A voidwalker can absorb the darkness surrounding it, thereby healing itself of wounds; A voidwalker in total darkness can spend time concentrating drawing the darkness in and producing a faint indigo light around its body. A voidwalker may consume shadows in shadowy illumination but heals only half as much as it would in total darkness. If the voidwalker takes damage while using consume shadows, it may lose concentration and fail in the attempt.
Most non-enslaved voidwalkers belong to a faction known as the Void, lead by Dimensius. Dimensius' forces destroyed the ethereal homeworld of K'aresh, and consequently, the Ethereum seek vengeance upon him at any cost.
The rogue voidwalkers seen in places like the Warp Fields notably wear what appear to be pauldrons, and sometimes breast plates, as opposed to bracers. The reason for this, and any difference it may make on their abilities, is unknown.
Perhaps the voidwalkers on Azeroth are "shackled" from their original plane, like elementals. It will explain the lack of bracers on the Outland-voidwalkers as Outland technically is in the Twisting Nether.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.
When a naaru dies, the light within them slowly bleeds away, leaving void energies in its place. These void energies draw in souls and turns them into creatures of the void while the naaru helplessly watches. This process of spawning these creatures is apparently necessary for, "without the void, the Light cannot exist".
It may be that voidcallers are thus born from dying naaru. Voidcallers have been seen in Outland nearby the darkened naaru K'ure, and the crystalline constellation over their backs hint at a connection with the dying celestial creature. According to K'ure, a naaru's body turns from pure light into a vortex of dark energies when it dies, from where these voidcallers could possibly arise. Another example of this is in the Sunwell Plateau, when M'uru's naaru form is defeated, he transforms into a massive void god known as Entropius. Voidcallers and naaru may also be directly related, as they have naaru-like crystals on their backs.
It is also possible that voidcallers are voidwalkers that have obtained these crystalline structures by feeding off the void energies of the dying naaru to empower themselves.