Essence

Essence is the genetic information carried by all life and is key to zerg evolution. The zerg take the essence of their prey, either through consumption or assimilation, and in doing so, incorporate their prey's desired strengths into themselves. This unique ability to steal essence motivated the fallen xel'naga Amon to enslave the zerg during their uplifting on Zerus. The zerg are said to possess "purity of essence," along with other species over time.

The amount of essence required for a successful evolution varies. For example, in order to evolve the creeper strain of the swarm host, only a single ash worm needed to be killed. However, in order to evolve the impaler strain of the hydralisk, multiple impaler colonies had to be destroyed before enough essence was collected. In other cases, essence from specific individuals of a chosen species - (e.g. an ursadon matriarch) must be collected to augment the Swarm appropriately.

The essence of each zerg type is stored in specific hive structures (such as the lurker den) and allows hives to spawn their various minions from larva. In the aftermath of Sarah Kerrigan's de-infestation and the subsequent fragmenting of the Swarm during the Second Great War, the zerg lost the essences of many of their strains. They were subsequently forced to seek out the lost strains amongst feral zerg, and re-assimilate their essence through capture or consumption. The corruptor's essence was completely lost and unrecoverable.

Some essence cannot be assimilated by the zerg&mdash; the protoss cannot be assimilated into the Swarm due to the protection the Khala and energies of the Void provide for Khalai and Nerazim respectively. Hybrid essence, while strong, cannot be consumed. Terran essence is considered to be unimpressive by Abathur. While some terran individuals can retain their psionic potential and intelligence post-infestion, they are the exception rather than the rule.