Dark Swarm (or Swarm for short) is a defensive spell cast by the Zerg Defiler. A buzzing, orange cloud appears on the battlefield, encompassing a 6x6 matrix area in which ground units take no damage from ranged weapons. Only melee attacks, splash damage, or special weaponry are able to affect a unit underneath Dark Swarm. The Dark Swarm lasts for approximately 38 seconds each time it is cast. Swarm will only protect units. Buildings in the cloud remain vulnerable to attacks by ranged weapons. Note that all ground units, including enemy units, are protected. 
The Dark Swarm cloud is slightly shorter and wider than the actual area of effect of the spell.
Although in nature a defensive spell, Swarm is often used offensively to give Zerglings and Ultralisks a devastating edge. Casting Swarm on the enemy’s army forces the opponent to retreat or die. Used defensively, Swarm can make it very difficult for the enemy to engage. Dark Swarm used in conjunction with Cracklings or Lurkers can be very effective at harassing enemies. Lurkers are particularly well-protected under Swarm, as Burrow prevents damage from splash attacks. There is however an exception. If a unit is directly placed above a Lurker and hit by splash damage, the damage will be also dealt to the Lurker underneath.
Even though high up the Tech tree, Swarm is often used in the late game phases of the ZvT match-up as its defensive properties are very effective versus the Terran army that consists mostly of ranged attack units in Marines and Tanks. Only Firebats will deal damage against burrowed Lurkers with their normal attacks. Yamato Cannon, Irradiate, Spider Mines and Nukes also work against burrowed units under the swarm.
Although Protoss usually incorporates Zealots and Archons, Swarm is still effective against Dragoon-heavy armies and Cannon-defended outposts. Late game ZvP often sees Swarm if gas-spendage and army composition allows.
Since all airborne units possess ranged attacks, Dark Swarm is a particularly effective tool against an attack from the air when Hydralisks are placed under the cloud. This combination is considered a good counter to Carriers.
In competitive play, this high-tech spell usually finds no place in this low-econ match-up.
In Combination with Other Spells
- Yamato Cannon, Spider Mines, Irradiate, EMP Shockwave, Lockdown, Psionic Storm, Maelstrom, Feedback, Plague, Ensnare, and Spawn Broodling will be unaffected by Swarm.
The map Persona has neutral permanent Dark Swarms on it that mostly cover ramps and protect mineral lines.
|Iris||Daum OSL Finals, Game 5 - Excellent use of Swarm by GGPlay|
|Unknown player||Yellow Swarm, Place 8 from Pimpest Plays 2003|
- adapted from: Zerg Strategy Guide