Dark Swarm
Cost: 100
Range: 9
Dark Swarm is created from a countless number of smaller creatures that typically cover the Defiler. The Defiler has the natural ability to launch these creatures to a projected position, creating a thick cloud of living insects that prevent any units outside from accurately targeting any units within the cloud.
The Dark Swarm covers a 6x6 matrix area and lasts approximately 60 seconds. Ground units within the cloud cannot be hit by ranged attacks from either ground or air, however this will not stop melee attacks or splash damage from affecting the units inside the Dark Swarm. Note that while ranged attacks will not affect units within the cloud, ranged attacks from units inside the cloud can still hit units outside of the cloud. Also, ranged attacks from units within the cloud will not work against other units inside of the Dark Swarm. To further clarify, listed below are a few strategies to use with Dark Swarm.
By launching a Dark Swarm over enemy ranged attack forces, defensive structures, or a projected battle area, you can give your own melee attack units (Zerglings or Ultralisks) a short period of invulnerability while attacking. A Zergling/Dark Swarm combination makes the Zergling much more valuable. A typical blockade of Bunkers or Photon Cannons can easily be overrun if they're covered with a Dark Swarm and attacked by Zerglings.
Dark Swarm can also serve as protection for your own Hydralisks from enemy ranged attacks. This is especially useful for countering fleets of Carriers or Battlecruisers or even forces of Dragoons, Marines, Goliaths, etc. One thing to watch for when using Dark Swarm on your own units is the enemy trying to sneak in to use it to their advantage. Keep your units at the edge of the cloud to get the most out of the units' attack range as well as to prevent enemy units from sneaking in. Should the enemy approach your Dark Swarm with melee attack units, do everything you can to keep them out. Once they manage to make their way in, they will be able to ravage your ranged attack units within the cloud and turn the advantage against you. If there is no way to stop an incoming melee attack group, retreat far enough so that the enemy is no longer protected by the Dark Swarm and resume the attack.
While Dark Swarm will prevent damage from ranged attacks, it will not protect from ranged special ability effects. Psionic Storm, Maelstrom, Plague, Ensnare, Irradiate, EMP Shockwave, etc. will all still be able to affect units beneath the Dark Swarm.
Terrans are especially vulnerable to Dark Swarm since every Terran unit, except for the Firebat, has a ranged attack and therefore are unable to attack any units within the cloud. A flood of Zerglings protected by a Dark Swarm can easily ravage a Terran base or force. With multiple Dark Swarms covering a large area, the Zerglings have an even greater advantage with free reign over the entire affected location. Be sure to eliminate Firebats and Siege Tanks in Siege Mode first to ensure the advantage they have with the Dark Swarm.
The Protoss, with their Zealots and Dark Templar, are better able to defend against a Dark Swarmed invasion. However, depending on the number of Protoss defenders, a flood of Zerglings protected by Dark Swarm can still be too overwhelming. Watch for Psionic Storms within the Dark Swarm, since they will still ravage hordes of Zerglings at a time. Also keep an eye out for Archons, as their attack does a small amount of splash damage even to units protected under a Dark Swarm.
Opposing Zerg broods can be difficult to use Dark Swarm against, since they can produce their own Zerglings in which case the Dark Swarm is useless. However, Dark Swarm can be used to protect your bases from Guardian/Mutalisk or Hydralisk attack but, once again, be sure to have lots of Zerglings at hand to really turn the tide of battle.
Here is a list of units that are NOT affected by Dark Swarm:
Terrans:
Firebats
Siege Tanks in Siege Mode (can still inflict splash damage)
Protoss:
Probes
Zealots
Dark Templar
Reavers (can still inflict splash damage)
Archons
Zerg:
Drones
Zerglings
Lurkers (can inflict both direct and splash damage)
Ultralisks
Broodlings
Infested Terrans (can inflict both direct and splash damage)
When facing these units, it is usually not worth it to use the Dark Swarm unless you have the numbers to overwhelm them.
One thing to note is that Dark Swarm does not affect buildings in any way or form. Should they be covered in a Dark Swarm, they will NOT be protected from ranged attack either from units inside or outside the cloud, they will take damage as normal.