From the introduction:
What are the Xel’naga?
The Xel’naga are a long lived race. In fact they may live longer than entire universes. They are not fully immortal though, which is why they created the Infinite Cycle of ascending purity of essence and purity of form into Xel’naga. Their form of reproduction. They seed universes with life and wait for a race of Purity of Essence and a race of Purity of Form to arise. These races will meet and peacefully merge into one. At this point, the eldest Xel’Naga would sacrifice their essence to ascend the leaders of the race to Xel’naga. The Xel’naga are the most psionically powerful race in the StarCraft universe. It isn’t an exaggeration when they say that Amon could rip worlds apart. For most normal Xel’naga the very presence of Xel’naga DNA is something that can induce the growth of life. Formerly barren planets became covered in flourishing forests within months of being exposed.