- Daxxarri on the effects of a psionic storm.(src)
Protoss who follow the Khala can produce such a storm by closing their mind to the outside world, sending out psychic "ripples" that are disruptive to other lifeforms, tearing apart their brains. Such storms can also damage vehicles and the landscape itself. Such is the power of a psionic storm that it can send shockwaves through the air around it.
The storm is a physical display of the psionic power of a high templar. It concentrates all the psionic energy over a given area, so overlapping storms have no greater effect than a single one.
While Dark Templar can create psionic storms, only through the discipline of the Khala can it be controlled. Khalai who have not mastered the use of the ability can try to control psionic storms unleashed by Dark Templar (and similarly-powered protoss, such as the Tal'darim) but it frequently results in the death of both participants as they burn out like a star. Individual protoss who have used both types of abilities (such as Adun) have faced a similar fate.
During the Discord, Dark Templar lost control of their powers, creating storms across Aiur which killed many protoss and destroyed parts of the landscape itself. Storms appeared on other protoss colonies, such as Khyrador.
Psionic storm is a high templar ability in StarCraft, and is their primary combat ability since regular high templar do not have a regular attack.
The high templar can create a storm of raw psychic energy. This is achieved by one closing their mind to the outside world, sending out psychic "ripples" that are disruptive to other lifeforms.
It does not prevent units from moving into or out of the affected area. Only one storm may affect an area at a time; overlapping storms does not increase damage output.
Psionic Storm deals 80 damage in the target area in small packets over time, and hits both air and ground units. It injures friendly units caught in the area of effect of 1.5.
Psionic storm is smartcast.
Psionic Storm is subject to a short delay before striking; during this brief period there is a visual indicator for where it will strike.