Infinite Cycle

The Infinite Cycle was a cycle of rebirth enacted by the xel'naga.

Origins
The xel'naga arose within the Void. While incredibly long lived by even protoss standards, the xel'naga were 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. The 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. Each time, the xel'naga observed and identified the potential of their offspring, but never interfered directly.

Broken Cycle
In what would be the last universe to bear witness to the cycle, the xel'naga seeded the universe with life through Ulnar. They entered slumber, waiting for the two destined races to find and assemble the Keystone, which would lead them to Ulnar, beginning the cycle anew.