Psionic Storm- Research at Templar Archives. Costs 200 minerals/vespene, and costs 75 energy to use.
Along with Disruption Web, Psionic Storm is one of the more powerful spells in Starcraft that is, once again, a weapon belonging to the Protoss armies. The High Templar possess this weapon of destruction, which effecively deals 128 damage over the course of four seconds to any and all units- friend or foe- in the area of a 4x4 matrix. It effectively slaughters any Terran Infantry brought against it, does great harm to the low-tech Zerg ground units, and deals significant, if not fatal damage to air units stacked on top of one another. When used against a player who does nothing but macro, it can and will win games.
This spell is most dangerous to Zerg players, who deal with units that usually don't have enough HP to be able to stand from a storm and thus have to manually redirect their units around or away from it. Terran players also suffer, but to a lesser extent due to the capacity of a well-placed Siege Tank's longer range. Psionic Storm is also used in the technique of Storm Raids, where an enemy's worker line can be decimated by a few well-placed Templar. However, Psionic Storm is not the end-all technique- the High Templar itself has no weapon, and is thus vulnerable to Terran EMP. A player can also bait his opponent into using the Storm and then quickly moving away, using up his opponent's energy and then moving in for the kill.