Psionic Storm (also known as Psi Storm) is an offensive spell cast by the Protoss High Templar. Psi Storm creates an energy storm in a target area, dealing a total of 80 damage to all units within the area over a 4 second period. It does 10 damage for every half second. The cooldown for this ability is 2 seconds.
In competitive play Psi Storm is often used to deal high amounts of damage to enemy forces that have clumped together, weakening or killing a large number of enemies at once. Psi Storm is the High Templar's primary spell, and makes the High Templar a target for the opponent to snipe due to the potential damage Storms can cause. Even if a Psi Storm does not kill all of the units that it hits, they will be significantly weakened. When used with the Warp Prism, a "Storm Drop" in the enemy's mineral line can interrupt mining operations and possibly kill workers.
While the Psi Storm is not capable of killing many Protoss units by itself, it can be a helpful and unexpected tool in weakening the opposing Protoss player's army. The most common use of the Psi Storm is versus Void Rays, in combination with Stalkers. Storm is effective versus Void Rays because they tend to clump up, which makes the storm deal damage to every ship. If the Void Rays' player manually split them then they will be picked off by the Stalkers one group at a time. In a battle without Void Rays the clearest target for the storm is the enemies ranged units like Stalkers and Sentries. Targeting Zealots can also work if the Zealots are not yet within melee range of the players units.
A High Templar's Psi Storm can lay waste to a Terran player's Marine/Marauder army, particularly if they are not microed properly. Similarly, fleets of Banshees and Vikings can be weakened by a Psi Storm. Though more risky but with potential for more damage is to wait for the Marines-Marauders to Stim, either to run away (kite) or to engage, before dropping storms on and in their path. 
Psi Storm, just as in SC1, is deadly against the Zerg in SC2. Because Zerg forces tend to come in waves and swarms, Psi Storm can kill many Zerglings at a time, and can reduce Hydralisks life to 1 if the opponent doesn't successfully micro. As with SC1, Psi Storm is also helpful in repelling Mutalisk attacks, but also makes the High Templar a priority target of fly-by Mutalisk attacks.
In Combination with Other Spells
- Casting multiple Storms does not stack the damage.
- Pinning enemy units with Force Fields and following up with Psi Storm can be very effective.
- Slowing down enemy units with a Mothership core using Time Warps and following up with Psi Storm and/or Force Fields can be truly devastating.
- A great time to cast Psi Storm is right when Vortex has worn off and all the enemies units are now in a tightly packed ball, but this only works in Wings of Liberty as Vortex has been removed in Heart of the Swarm.
- Affects friendly units
- Affects both ground and air units
- Affects cloaked/burrowed units
- Does not affect buildings
|Patch 7 (version 0.9.0.14621B)|
- Fenix (T) vs MaNa (P) Game 1 - DreamHack SteelSeries Tournament Ro4 - Excellent Psi Storm usage (Day.tv)