In 500 B.C. (approx.), Savassan and Temlaa discover the Khaydarin Crystals. Savassan realises how they have the power to unite the protoss race through a telepathic link and proceeds to do so along with the Shelak Tribe. This is the start of the Khala and the beginning of the end of the Aeon of Strife.[1][2]
With the kindling of the Khala, the xel'naga of Ulnar are awoken and travel to Zerus. Amon's xel'naga and the Ulnar xel'naga come into conflict. In a long and brutal battle over Zerus, Amon unleashes the zerg swarm on the Ulnar xel'naga, killing a majority of them. What few remain still manage to cast Amon back into the Void, but at a great cost. The survivors go back to Ulnar to rest, waiting for the next host creature to awaken them.[3]
In Resurrection IV, Taldarin claims he fought beside Adun 3000 years ago, which would place such an occurrence at round about this time. However, Adun died in around 1500,[2][5] which would make this timeframe seem implausible, not to mention it marking the beginning of reunification rather than its middle. Chris Metzen later clarified the issue by explaining that the comments were written at a time when the timeline was still being pinned down and that Taldarin fought much later.[6]