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Ulnar (or The Temple of Unification) is a massive xel'naga temple located in the Alterian Rift. It was the resting place of the xel'naga, and served as a portal leading to the Void. A massive amount of Void energies emanate from Ulnar.
Cradle of LifeEdit
Ulnar served as the temple from which the xel'naga seeded the universe with life. It was located between dimensions, and served as a portal to the Void. After they had finished, the xel'naga rested within the temple, but not before creating an artifact known as the Keystone. The Keystone was designed to find its way into the hands of the ascended host races, races that would naturally become gain both purity of form and purity of essence, and guide them to Ulnar. It would then release the energy it had stored, awakening the xel'naga. The elders among them would give their essence to the chosen races, and they would ascend as the new xel'naga.
This plan went awry when Amon and his followers started to manipulated the Infinite Cycle to their own ends, and personally took a hand in uplifting the protoss and zerg. The creation of the Khala awakened the xel'naga in Ulnar, who fought him and pushed him back into the Void, but at a high cost. The few that remained returned to their slumber. Ulnar took on mythical proportions within protoss culture as a realm of myth; the place from which all life flew.
Upon Amon's return to the material universe, he entered Ulnar and killed the xel'naga slumbering there. Sarah Kerrigan would send her Zerg Swarm to the temple only to find Amon's hybrid inside. She and her broods would battle them for weeks.
The Daelaam would arrive on Ulnar after discovering the location locked in the Keystone. Upon arriving they attempted to activate the Celestial Locks sealing the main entryway, but the Tal'darim and Moebius Corps attempted to stop them. The Daelaam broke through, and Artanis entered the temple. He met with Kerrigan, and the two fought through Amon's forces and the xel'naga constructs in order to discover the history of the xel'naga and the infinite cycle. However, when they arrived at the Chamber of Ascension, they found the xel'naga dead. Amon sprung his trap and opened a portal to the void within Ulnar. The combined forces of the zerg, Daelaam, and Alarak's Tal'darim destroyed the void crystals powering the portal and sealed it.
Later in the war, Kerrigan gathered her allies for an attack on Amon directly in the Void. Ulnar opened itself up as a massive portal, allowing their capital ships to attack Amon in his own realm.
When Kerrigan and her allies returned to Ulnar, Amon's forces attempted to destroy Ulnar by overloading the Celestial Locks, to prevent them from entering the Void. Allied forces under the advisement of Rohana stopped the attack and prevented Ulnar's loss.
In Wings of Liberty, mission description data points to a distant, shadowed world as being Ulnar, and is likewise named as such in the game's strategy guide. Legacy of the Void depicts it as being an artificial construct. Concept art for the game depicts Ulnar as having a planetary surface.