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 infestation, and in doing so, incorporate their prey's desired strengths into themselves. This unique ability to steal essence led the fallen xel'naga Amon to enslave the zerg during their uplifting on Zerus.

The amount of essence required for a successful evolution varies depending on the prey species. 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 killed before enough essence was collected.

The essence of each zerg type is stored in specific Hive structures (such as the Lurker den) and allows the Hive Clusters 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.

Some essences cannot be assimilated by the zerg. The protoss are one example.