The protoss believe the xel'naga used the crystal to enhance the rate of protoss population growth. The crystal was sealed with xel'naga energies in a temple. The Velari Templar School was built over it. The route there required passing through shifting catacombs known only to very few templar, such as Lekila, a teacher at the school.
The Fall of Aiur
The Velari Templar School was lost on Aiur during the zerg invasion. Lekila led a group of promising students from the school, but was attacked by the zerg. During the battle, her psionic appendages were severed. She was rescued by Rihod, an old friend.
A force of protoss returned to the planet to reclaim it, among them high templar Lekila and Rihod. The site was under siege by zerg, which also sought the crystal, requiring the protoss force to fight past and evade the zerg. As Lekila tried to navigate the difficult catacombs, the protoss transport ship fell under attack by the zerg.
Lekila discovered the crystal, but the xel'naga had sealed it against Khala energy; only Dark Templar Void energy could dislodge it from its setting. Lekila, having lost her psionic appendages, was able to channel the Void, and did so to take the crystal. The zerg attacks threatened the protoss force and ship; Rihod and Lekila merged into a twilight archon to combat them and teleported the Kassia crystal safely aboard an orbiting protoss vessel, where it was taken to Shakuras.