Void

"This is something born of the void. Cold and empty. But so powerful it hurts."

- Sarah Kerrigan The Void is a realm separate from the material universe.

The Dark Templar
"The Void is not like the Khala. It does not draw us together and connect us in a blanket of warm feelings. The Void certainly does not offer itself willingly. A dark templar stalks the Void and, through will and discipline, forces it to give up its power."

- Theromos

The Void is understood by the Nerazim. Due to severing their nerve-appendages in a display of contempt for the Conclave, they lost the primal link the rest of the protoss share, and removed them from the control and discipline the Khala provides. Forced to live in secrecy to avoid detection by the Khalai, and without the psionic focus afforded by the Khala, the Nerazim were forced to delve into the mysteries of the Void, eventually learning how to wrest its power by means of rigorous training, discipline, and will.

The Void provides the Dark Templar with psionic powers unheard of by their brethren and grants the Dark Templar enormously strong individual will. They require great will and discipline to make use of the Void, and can be consumed by it if they do not possess these traits. Those who walk this path do so with diligence and reverence, for they know full well the dangers lurking in the shadows of the Void.

The "currents" of the Void allow faster-than-lightspeed travel for the Dark Templar. These currents are affected by gravity and solar winds. Strands of the Void can be wielded by a Nerazim, separating it into numerous strands if they so desire.

Void powers enable Dark Templar to resist infestation by the zerg. The particle disrupters mounted on stalkers use the power of a Nerazim to shoot entropic Void energy at targets; Tal'darim stalkers' have been observed to fire green projectiles, as opposed to the usual blue fired by their Daelaam counterparts.

The warp blades utilized by Nerazim warriors are infused with Void energy.

Culture
It appears that the Dark Templar associate the Void with death or the afterlife – to "embrace the Void" is to embrace death, so to speak. They consider anything that taints the Void to be unholy, and it is a major part of their religion. When the Nerazim began studying the Void they wrote texts on the subject, said texts becoming the basis of their teachings on the subject centuries later.

In recent times, certain Dark Templar have mentioned "an ancient voice whispering from the Void," chilling them to the core.

The Void and the Khala
Utilizing the Void and Khala simultaneously grants protoss incredible power, but can result in the protoss burning out like a star. Protoss who can utilize both sets of power simultaneously are exceedingly rare. Known practitioners were Adun and Tassadar. Similarly, the prismatic core of the void ray draws upon the powers of the Void and Khala in tandem, forming a self-sustaining reaction and thus making the core a virtually eternal power source.

The Tal'darim
The Tal'darim believe that Amon resides within the Void, and it is only through His blessing that one is able to travel through it. It is from the Void that they draw energy for their psi-blades.

Other Races
The Void is inhabited by a mysterious entity at KL-2 calling itself the "Heart of the Void". The KL-2 entity "tainted" the Void, according to the Dark Templar.

The xel'naga understood the Void. Their ships possessed the ability to travel through it. The xel'naga imprisoned the KL-2 entity using the powers of the Void. When their physical bodies are killed, their minds survive within the Void, and they can be revived with enough psionic energy, as happened with Amon.

It appears that the Overmind knew of the Void in some form or another, as the powers the dark templar wield were similar to its own, such powers being the only thing that could permanently destroy its cerebrates and the Overmind itself.