The Defiler is late game unit requiring the Hive and Defiler Mound, at which time it can be morphed at any of the Zerg player's Hatcheries, Lair, or Hive. It does not deal any damage directly, thus is easily susceptible to opponent attack. As such, Defilers usually travel along with Zerg armies as a support unit rather than traveling individually. Despite this, Defilers are relatively slow and move in an inelegant slither, and thus easily fall to the rear of the Zerg forces. To mitigate this, it is common for the Zerg to transport Defilers in Overlords and as part of a drop attack. Defilers are most commonly used against Terran and Protoss, against whom their spells are most effective.
Zerg players usually use the Dark Swarm spell to defend against an army that is composed mostly of ranged units. The Dark Swarm does not block spells such as Psi Storm and certain attacks such as those from melee units like the Zealot. Under the Dark Swarm, the attack of all ranged units are shifted to the front of their intended target (i.e. falling short), so splash damage will still deal damage.
The Defiler is frequently sniped by enemy units due to its potent spells and slow movement speed, so it's common to see the Zerg player hide the Defiler in the center of its army force or using Overlords with speed/drop upgrade to quickly get the Defiler out. It is very similar to Reaver/Shuttle combo that Protoss players frequently employ.
Although rarely seen in Zerg vs. Zerg games because these match-ups rarely reach the Hive stage, a Defiler can single-handedly swing the momentum in a player's favor with one well placed Plague. Because ZvZ games are generally limited to Zergling and Mutalisk play, a Plague over a group of the enemy's stacked Mutalisks can completely debilitate them.
Defilers are most commonly used versus Terran because Firebats, Siege Tanks in Siege Mode, and Spider Mines are the only units capable of dealing damage to units protected by Dark Swarm. And even in those circumstances, Siege Tanks will only deal splash damage to the target, thus being able to sustain several shots.
The best time to use Dark Swarm is usually to protect a small area of the map, either offensively or defensively. This includes attacking enemy-controlled bases or choke points, or defending the Zerg player's own chokepoints. It is common for the Zerg to utilize Mutalisks to stall the Bio Terran in the early game as the player quickly advances to Defilers with Dark Swarm, which renders the Marine and Siege Tank army largely ineffective. Although it is also effective in more neutral engagements (such as two armies clashing in the center of the map), Terran players will often move their troops from under the Dark Swarm to minimize damage. Lurkers under Dark Swarm are extremely powerful. Because they are burrowed, Siege Tanks do minimal damage to them, and Firebats' concussive damage makes them very weak against Lurkers.
Plague is also very useful against tightly packed units like Marines or against Science Vessels. Plague is best used when the Zerg player's army is located away from the Defiler while the Terran opponent's army can be targeted. Any Zerg units hit by the Plague spores will also see their health severely damaged. A Plagued infantry army becomes immensely fragile since their Hit Points will sit at 1 for a while even as any nearby Medics attempt to heal them. When used on clumps of Science Vessels, this either leaves them extremely vulnerable, or forces the Terran to retreat and repair them. Plague amplifies the power of Hydralisk/Lurker against SK Terran as the Hydralisks are very effective at picking off a Plagued Terran army as it retreats. These troops will each only require one Hydralisk or Mutalisk attack to complete be destroyed.
Because of their potent spell casting abilities, Defilers are often the first priority for Science Vessels to Irradiate. One Irradiate is sufficient to kill the Defiler, but the Zerg player will have a few seconds to cast a last Dark Swarm or Plague for defensive or offensive purposes before expiring.
Defilers are somewhat popular versus Protoss, although players will usually wait until they have about four mining bases until they start producing them. Generally, they are more popular in the professional scene than among amateurs as they require additional multitasking and micro management to properly control.
Dark Swarm is particularly effective when used against a Protoss army consisting heavily of Dragoons or Archons, as the Dragoons get nullified due to its range attack and Archons are reduced to splash damage as its most potent attack is its short-range fire. Dark Swarm is also very effective at destroying isolated expansions. In the mid-game, Protoss often rely on Photon Cannons alone for defense for the Nexus, but they deal zero damage to units under Dark Swarm, so even a small group of Zerglings can destroy a large group of cannons if the Protoss has nothing else to defend with.
Plague is powerful against tightly packed ground units with high Hit Points, such as Zealots and Dragoons. Note the Plague is not effective against Archons, as most of their health is composed of its Plasma Shields.
If the Protoss elects to build Carriers, the Defiler's Plague is an effective counter, as Carriers are usually stacked. Additionally, Plagued Interceptors will stay in the Carrier until Plague wears off. This is because Interceptors will return to the Carrier to heal. Carriers are also countered by Dark Swarm with Hydralisks, as Hydralisks can fire from under the Swarm without incurring damage from Interceptors.