Purity of essence

"My children, the hour of our victory is at hand. For upon this world of Aiur shall we incorporate the strongest known species into our fold. Then shall we be the greatest of creation's children. We shall be... Perfect."

- The Overmind

Purity of essence is a concept first pioneered by the xel'naga. Standing in contrast to purity of form, the xel'naga applied this concept when choosing a species to uplift to merge with another, in order to rejuvinate their race in their cycle of reproduction. In the case of the Milky Way, it was the zerg that were deemed to have this trait and over the course of their evolution on Zerus, were deemed to retain it.

The quest for 'purity' was retained in the zerg psyche, becoming a pseudo-religeons concept to them. Through the quest for 'perfection' (at first by the Overmind and then the Queen of Blades), the zerg were set on a steady state of evolution and conflict. The core idea of this 'religeon' was that there was a state that the zerg could reach where they no longer needed to evolve, that their evolutionary form would never have to change again because they could already adapt to any situation. Abathur, an evolution master, doubted that this was possible, but reasoned that chasing perfection was tactically sound regardless.