The defiler is a late game unit. It has no direct attack and is slower than other zerg ground units; it is a support/harassment unit. Using multiple defilers makes it easier to manage the simultaneous deployment of Dark Swarm and Plague.
Dark Swarm casts a cloud over an area which prevents both friendly and enemy units (but not structures) under the cloud from taking damage from ranged attacks. Units under the cloud may still be damaged from splash effects. Lurker, firebat, and reaver attacks are not affected by the cloud. Units outside the cloud may still be hit from ranged attacks originating from inside the cloud.
Dark Swarm may be used to cover the advance and attack of melee attackers, or shelter ranged attackers from return ranged fire.
Plague has a smaller area-of-effect than Dark Swarm. It inflicts up to 295 damage, to a minimum of 1, over time to each affect unit. Plague affects units and structures. Plague does not affect shields; damage is applied directly to the hit points underneath. It is especially useful against the protoss because their HP cannot be restored.
Defensive lines and other enemy concentrations may be softened up with Plague prior to an assault.
As a support unit, burrowing keeps the defiler out of harm's way while its energy recharges. It may be used to set up ambushes using Plague, or help position the defiler to cover the arrival of friendly forces with Dark Swarm.
Defilers are an unarmed caster unit, so they are vulnerable to any attacking unit, especially faster air units that it would not be able to run from very well. Specific hunter units are particularly deadly such as vultures and their spider mines and groups of wraiths.
Defilers can burrow to help hide them from these vulnerabilities.
Shields units in a 6x6 area from ranged attacks. Ranged attacks with splash damage only deal minimal splash damage. Firebat and lurker attacks, and damage from cast abilities, are totally unaffected. The casting player's buildings are shielded; enemy and allied buildings are not.
Sprays all units and buildings in a 3 x 3 target area with spores that inflicts 25 damage per second for a maximum of 295 damage. 'Plague' will not kill or destroy affected units and buildings leaving at least 1 hit point. Plague does not drain shields but will affect the hit points underneath regardless of shield strength. Affected cloaked units will be revealed.
Kills a "friendly" zerg unit while giving the consuming defiler 50 energy points. The amount of energy gained is the same regardless of the unit 'consumed'. Therefore a cheap unit, such as a zergling, or one that is about to die, is considered the best "food".