The Xel'Naga are an interdimensional species of extraterrestrials in the StarCraft universe.
Originating from the void, the Xel'Naga were incredibly long-lived even by Protoss standards, but not immortal; eventually their species would come to an end. In the interest of survival they planned to create new Xel'Naga by uplifting two species. A pair of qualities was required and these were defined by the Xel'Naga as purity of essence and purity of form, and each uplifted species would be targeted for one of them. Over an extraordinarily long period of time, the two species would naturally come together and merge, forming the next generation of Xel'Naga and beginning the cycle anew. This cycle was repeated in numerous universes; at the start of a universe's creation, the Xel'Naga would take on physical form and enter the universe, seeding it with life. The cycle repeated so many times that time itself was affected by it.
During their journeys, they learned of the Voice in the Darkness, a Void-based entity which consumed entire civilizations. The Xel'Naga, who protected life, imprisoned it with an Argus crystal beneath a barren world at KL-2.
Driven to perfect their science of proto-genetic evolution, the Xel'Naga focused their frustrated efforts on Aiur, one of their engineered worlds. Pleased with the progress of its most advanced native life-forms, the Xel'Naga conceded that their latest creation was their first of all their experiments to evolve beyond the feral constraints of baser life-forms. They gave the new race the name of Protoss, or 'the firstborn'.
Ouros was one of the Xel'naga who helped construct the material universe and seed it with life.
Amon (or the Dark Voice, the Fallen One, the Dark One, the Dark God, the Eternal One, the Master) was a malevolent Xel'naga. He sought to break the "corrupt" cycle of the xel'naga and bring all life to an end, before remaking it in his image. He has been described as "the universe's most ancient evil."