Dark Swarm
Cost: 100
Range: 9
Dark Swarm appears on the battlefield as an orange smoke cloud. The cloud encompasses a 6x6 matrix area. Units within the cloud take no damage from ranged attacks. Only units with melee, splash, or special weaponry are able to affect a unit underneath a Dark Swarm. Note that the Swarm will only protect units, and any buildings in the cloud are still vulnerable to attack by ranged weapons.
Since all airborne units possess ranged attacks, Dark Swarm is a particularly effective tool against an attack from the air when you place Hydralisks under the cloud. When used as part of an attack or defense on the ground, care must be taken to prevent enemy units from also using the Dark Swarm as protection. The Dark Swarm can also protect enemy units from the ranged attacks of your own Hydralisks, Mutalisks, and Guardians. Position your own ground forces to block the path of the enemy troops into your protective cloud.
The following units are able to damage units underneath a Dark Swarm with their normal attacks:
Protoss: Probes, Reavers, Dark Templar, Zealots, Archons
Terrans: SCVs, Firebats, Siege Tanks in Siege Mode
Zerg: Drones, Zerglings, Lurkers, Ultralisk, Broodlings, Infested Terrans
Many special abilities are also completely unaffected by the presence of a Dark Swarm including: Yamato Cannons, Spider Mines, Irradiate, EMP Shockwave, Psionic Storm, Maelstrom, Feedback, Plague, Ensnare, and Spawn Broodling.
Quelle:
http://www.battle.net/scc/zerg/special.shtml