Giant crystal

An unnamed giant khaydarin crystal was found at the heart of the Aiur chambers by protoss explorers.

A xel'naga-crafted artificial intelligence, its directives were unknown. The protoss mystic Khas believed it was designed to teach.

The crystal generated a powerful "heartbeat" sound in the chambers during the Aeon of Strife. It pulsated as it moved languidly and its facets were in a perfect ratio. The preserver Zamara opined it was made of very pure khaydarin crystal.

The Mystics
The crystal was discovered in chambers beneath Aiur during the late Aeon of Strife by the protoss mystic Khas and his student, Temlaa. The two had already encountered a number of "drained" protoss corpses on platforms during their explorations.

Eventually Khas came up with a theory; over Temlaa's objections, he removed one of the corpses and put himself onto its platform. He ordered Temlaa to use the technology to hook him to the machine. It began draining him until Temlaa reversed the orders.

When Khas came to, he had gained some knowledge of the xel'naga and how to end the protoss racial madness, eventually forming the theory of the Khala and reshaping protoss society.

Following a brief investigation, the Conclave ordered the caverns sealed and forbade entry into them.

Dark Future
Thousands of years later (and four years after the Brood War), the protoss had abandoned some of their kind on Aiur. These protoss split into groups; the Shel'na Kryhas (traditionalist Khalai) and the Tal'darim (a splinter group which was mentally regressing to the madness of the Aeon of Strife).

The latter group was led by a benefactor (secretly a dark archon named Ulrezaj), whom they knew as the Xava'tor. This benefactor took control of the Tal'darim using an addictive drug known as Sundrop, which separated the Tal'darim from the Khala and caused them to become mentally similar to the Dark Templar. Without even realizing it, the Tal'darim began to learn Dark Templar powers such as cloaking.

The Xava'tor taught the Tal'darim how to trap zerg, but in return he expect them to obey his orders. He forbade any Tal'darim from entering the giant crystal's chamber on pain of death, and would sometimes take a few protoss with him, calling them Xava'kai (his "hands"), and said he would have them perform secret tasks. In fact, Ulrezaj was sacrificing them to the giant crystal at a high rate.

The Raid
A trio of fugitives - terrans Jake Ramsey (an archaeologist) and Rosemary Dahl (a mercenary) - plus the protoss preserver Zamara (who was a disembodied spirit trapped within Ramsey's mind) had come to Aiur on a mission. Zamara kept her aims secret; her presence was inflicting brain cancer upon Ramsey, and she intended to remove an extremely pure piece of khaydarin from the crystal and have the Dark Templar based on Shakuras place her into the piece of crystal. On Aiur, the group quickly became part of the Shel'na Kryhas.

The caverns were guarded by the Tal'darim, and it was only after setbacks (and the assistance of Alzadar, a former Tal'darim) that the group was able to stealthily visit the giant crystal's chamber.

Underneath the crystal they saw six vats inhabited by strange psionic creatures, but the group was able to remove a piece of crystal without disturbing them. During the escape, Alzadar discovered the drained corpses of several Xava'kai in the outer chamber, which set off an alarm.

After confronting and escaping the Tal'darim (and then zerg sent by Infested Kerrigan, and a Terran Dominion fleet dispatched by Valerian Mengsk) the group fended off Ulrezaj long enough to escape through the warp gate to Shakuras, bearing with them the removed piece of crystal.

Abilities
The crystal produced a loud heartbeat sound during the Aeon of Strife, guiding explorers to its chambers.

One of its functions was "draining" protoss for unknown reasons. The draining could be stopped and even reversed, but was painful and would eventually kill the victim. A survivor of this process could awake with new knowledge from sharing things with the crystal.

It glowed magenta when draining energy, and glowed blue when restoring an injured protoss.