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Purity of form is a concept first pioneered by the xel'naga. Standing in contrast to purity of essence, the xel'naga applied this concept when choosing a species to uplift to merge with another that possessed purity of essence, in order to rejuvenate their race in their cycle of reproduction. In the case of the Milky Way, it was the protoss that were deemed to have this trait as per their excellent physical traits, prompting the xel'naga to guide their evolution. However, they pushed protoss evolution too quickly, which eventually led to the Aeon of Strife. The xel'naga reflected that they had perhaps marred the purity of their creations, and left the planet. However, the protoss would nonetheless suffice when the time came for them to merge with a species that was said to have purity of essence...
The protoss eventually emerged from their kinstrife and came to terms with their existence. They accepted that their ego had corrupted their racial essence, and that they were indeed a failed creation. However, as that inherent failure was not of their own doing, they were able to look to the future.