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 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.
In addition to using newly acquired essence to produce new strains, the Swarm 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.
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.
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. Abathur noted that the swarm host's essence constantly adapted, rejecting even simple mutations. The overlord's essence is 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. Terran essence is considered unimpressive by Abathur. While some terran individuals can retain their psionic potential and intelligence post-infestation, they are the exception rather than the rule.
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. 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.
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.