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 a "purity of essence," along with other species over time. On Zerus itself, essence can be found in some bodies of water. According to Zurvan the first zerg were born when a single essence split into many. Within the first spawning pool, these essences fought and devoured each other and grew stronger, eventually giving rise to the zerg.
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 larvae. 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 strains. They were subsequently forced to seek out the lost strains amongst feral zerg, and re-assimilate their essences through capture or consumption. The corruptor's essence was completely lost and unrecoverable for the remainder of the Second Great War.
Uses and PropertiesEdit
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.
New essence can be acquired without assimilating another species. During the zerg invasion of Korhal, Abathur used the side effects of the Terran Dominion's experimental weaponry to evolve two new strains of ultralisk. New strains can also be developed through repeated cycles of natural selection, as demonstrated by the creation of the vile roach and the hunter baneling.
In addition to using newly acquired essence to produce new strains, the Swarm, via the hive mind, can rapidly manipulate the essences of any currently existing zerg troops on a wide-scale at any time. This enables the zerg to quickly adapt to different battlefield scenarios (such as Kaldir's flash freezes). In some cases, deceased zerg can be revived (or "reconstituted") so long as enough essence is available. Powerful primal zerg, such as the primal Queen of Blades and Dehaka, can "twist" essence to heal themselves and nearby allies.
Difficulties Associated with EssenceEdit
Some essences cannot be assimilated by the zerg— 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. Protoss genetic material itself cannot be used by the zerg. Hybrid essence, while strong, can also not be consumed. After the End War, Overqueen Zagara, in accordance with Kerrigan's wishes, forbade the Swarm from assimilating xel'naga essence that had been recovered from Ulnar, and instead chose to create non-Swarm controlled creatures capable of housing the xel'naga essence.
Other essences, even after being successfully assimilated, can prove difficult to work with. For example, Abathur noted that the swarm host's primal essence constantly adapted, rejecting even simple mutations. In other cases, different essences will compete with one another (such as the Raptor and the Swarmling), making it impossible to integrate them into the same strain. Psionic essence has always posed a challenge for the zerg. The overlord's essence remains virtually identical to that of the original Gargantis proximae. Dr. Talen Ayers theorized that the zerg did not make any significant changes to the Gargantis' genetic code in order to preserve their psionic abilities. And despite the Swarm having originally targeted Humanity specifically for its psionic potential, Abathur considers Terran essence unimpressive. While some terran individuals can retain their psionic potential and intelligence post-infestation, they are the exception rather than the rule.
Energy Based EssenceEdit
The essences of the primal pack leaders on Zerus are visually akin to purple-white energy. After being reborn as the primal Queen of Blades on Zerus, Kerrigan slew the pack leaders and absorbed their essences.
Xel'naga essence is visually akin to golden energy. To the xel'naga, essence, matter, information and consciousness were interchangeable, and could be converted through relics such as the Keystone. In accordance to the Infinite Cycle, the elder xel'naga would sacrifice themselves to transfer their essence to their chosen offspring, transforming them into xel'naga themselves. During the final stages of the End War, Ouros imparted the last of his essence to Kerrigan, transforming her into a xel'naga and giving her the power to destroy Amon. After the End War, xel'naga essence was limited, and used to create the adostra and the chitha. Creatures influenced by xel'naga essence exhibited patterns of the Cuccodujo series throughout their biology.
Essence is collectable in some Heart of the Swarm missions, allowing Kerrigan to level up. In the mission "Harvest of Screams," the player must collect the essence of several Ursadon matriarchs to make their forces immune to Kaldir's flash freezes. Many evolution missions also involve collecting essence.