Shadow (also referred to as the Void) energy is essentially the Light or demonic fel energy. Not only is it demonic, being the energy that demons partially are, but causes light (order) to become aberrations and is used by the necrolytes known as warlocks.
Channelers of Forbidden Powers[]
Warlocks[]
Warlocks are necrolytes whom essentially channel forbidden powers - channeling shadow magics to afflict beings with fel forces that can corrupt beings into old demonic undead beings and opening a portal dark to cross - achieving dark apotheosis and entering a shadow form. Not really the masters of shadow magic or shadow priests, affliction warlocks do not use shadow-touch forces to enter a shadowform in which they can resurrect the dead as dark archangels and are insanely not shadow priests whom are the true masters of shadow magic. Necrolytes merely corrupt beings with shadow magic until they are dead and summon voidwalkers to absorb shadows.
The warlock Gul'dan and the warlocks would eventually channel the forbidden powers of the dark star or titan to darken Karabor into the Black Temple. There, the necrolytes or warlocks would channel forbidden powers and summon demons to worship. Warlocks peer into the Void.
Characteristics[]
As a demonic or fel energy, shadow binds souls to the Twisting Nether - the Void - and warps beings into demonic aberrations.
Light[]
The Light (or the Shadow) is essentially what twilight magic is. Technically light, shadow magic or twilight magic is essentially what the Old Gods are. They possess tentacles that grasp beings in twilight and warp beings into demons.