Eldrun is a very old Twilight Templar scholar, and the father of Patriarch Darr. He is most notably accredited with the creation of the Ihan Crystal, and the discovery of the fundamental knowledge that formed the basis of Dreamspeak.
Currently, Eldrun watches over the Templar Archives on Shakuras.
Dark Templar scholar Eldrun, was born on Aiur and raised as a Preserver at the Sanctum of Memories. As a Preserver, he was trained to use the universal psionic link that connected all Protoss at the time known as the Khala, to access the memories of dead Protoss. Although the Khala was the source of his power and his responsibilities, and had ushered in a new era of peace for the Protoss following the Aeon of Strife, Eldrun was not sure he was willing to embrace the new doctrine, as he felt it destroyed the individuality of its followers.
As the years passed, Eldrun learned of a small but growing group of Protoss that had rejected the Khala. These dissidents, known as the Rogue Tribes or the Dark Templar, feared the Khala in a similar manner to Eldrun. He learned that, although the rogues held no ill-will toward their brethren and attempted to live their lives unnoticed as best they could, their existence was still known to the Conclave.
Eldrun began secretly meeting with the scattered members of the rogue group, and spoke with them often over time, mostly bringing news from 'the outside'.
As a Preserver, Eldrun was called upon to preside at the occasional meetings of the Judicator Caste, and as such, it was not long before he learned that the Conclave, under the leadership of Kortanul, had collected records on the locations of the Rogues.
(Eldrun was still very young at this time, only 246 years of age.)
The Conclave ordered the Executor Adun to report to the Conclave so they could tell him about this new threat, which they believed could cause a new Aeon of Strife. Adun was given Raszagal, a young prisoner, to question. He brought her to the Citadel of the Executor so the other Templar could question her. The Conclave gave him their information so he could quietly track down the rogues, abduct them, and execute them.
Shocked at the decisions of the Conclave, Eldrun spoke out of turn against their plans to destroy the rogue templar, declaring that it was unethical to kill so many of their brethren over their beliefs. The Conclave not only dismissed his words, but threatened to do the same to "those that sympathized with heretics". Eldrun was forced to resign from the meeting, fearing the worst for his rogue brethren.
After Eldrun had left, the members of the Conclave were more than suspicious of his words. In his haste to denounce the Judicators' plans and speak reason to the Conclave, Eldrun had uttered a fateful word... "Us".
This, in tandem with his utterance against them, caused them to believe that he too was a member of the dissident rogue templar, and they therefore ordered that Eldrun to be brought before the conclave for execution as well.
Adun was unable to bring himself to execute the Rogues, and so he transmitted fake recordings of the executions to the Conclave and hid Eldrun and the Rogues, even teaching them how to bend light around themselves to cloak. The Rogues ended up developing these abilities more quickly than Adun could teach them. Unfortunately, the Rogues, still learning their powers, lacked the discipline of the Khala. They inadvertently unleashed psionic storms, which spiraled out of control on Aiur. The Conclave discovered the corpses of dead Rogues, only to realize these Rogues had been depicted in the execution recordings.
The Conclave could neither punish Adun nor slay the Rogues, as he would force them to publicly admit the existence of the Rogues. The Conclave decided to banish the wayward Rogues from Aiur forever on an ancient, but functional xel'naga ship, along with other protoss ships which Adun insisted they be allowed to take with them. However, last-minute squabbling threatened the Rogues, and Adun feared they might accidentally create psionic storms again. Adun created a blue mist with a combination of Khala and strange energies, which covered and protected the Rogues. However, the power caused Adun to burn out like a star.
Shortly afterward it was discovered that the exiles had stolen the powerful Khalis Crystal from the Sargas Tribe and taken it with them.
Eldrun had used his influence to accomplish this.
Inevitably some knowledge survived on Aiur and over time the rogues became the subject of myth and legends. Eventually they became known, in legend, as the Dark Templar.
Eldrun journeyed through the stars with the Dark Templar for a long time, hundreds of years, contemplating what powers could be drawn from the ancient crystal. During this time the habit of cutting off their psionic appendages became common, but Eldrun did not practice this ritual, as those that did, in order to use psionic powers, were forced to draw energy from the void. In time Eldrun decided that in order to preserve this psionic link as well as their connection with the past, he would need to introduce his own philosophy to the Dark Templar for them to consider which he called the Boros Philosophy, meaning "They that remember". As time went on, the Dark Templar began to adhere to various ways of life proposed by their elder members, although the most prevalent philosophy was that of the Lenassa.
The first step on the Dark Templar's journey was the moon of Ehlna, which they discovered shortly after leaving Aiur. It had a xel'naga-crafted warp gate, so the exiled protoss stayed. Eldrun discovered the moon had a combination of energies which altered khaydarin crystals, making them quite efficient at storing memories. Without preservers, the Dark Templar had no other way to store memories, so they used this technological means. Eldrun was fascinated by these crystals and their powers of preservation, and he devoted all his time on Ehlna to their study.
During his studies of the self-named 'Ihan Crystals', Eldrun fell in love with a Dark Templar scholar named Aszhala, and together they sired a child, whom they named Darr, meaning "Honor". After more than two centuries, the Dark Templar departed, but some remained to operate a shrine, the Alys'aril, where the memories could be extracted from pilgrims and stored. Numerous vessels remained behind as well.
Eldrun took his family and left the planet behind but the discovery left him with the unsettling realization that, without connecting himself with the Khala, he could not access new memories.
The Dark Templar eventually discovered the dark planet, Shakuras, during their journeys and remained, due to the presence of a Xel'Naga temple. They found a warp gate which could be used to travel between Aiur and Shakuras as well as other worlds.
Eventually Raszagal, (age 545), one of few Dark Templar who was old enough to clearly remember Aiur, became the Matriarch of the Dark Templar. She ruled for approximately five hundred years before the outbreak of the Brood War.
Because the psionic link of the Khala is required to become a preserver and access the collective memory, non-adherents of the Khala, such as Eldrun and the Dark Templar, were unable to be preservers. This knowledge haunted Eldrun and so, he buried himself in deep study and research, constantly examining and re-examining the Khalis Crystal.
Although his studies offered him great knowledge and exemplary control over the energies of the Void, he was not any closer to uncovering his ultimate goal: the power to give the Dark Templar preservation.
Eldrun began to lose hope. He was very old by this time and feared he would never accomplish his goal.
Then suddenly, Eldrun began to see results, although not what he had expected, he believed that his findings could be gateway to his dream's realization.
Eldrun had found a way to access the psionic essence of dead protoss through the use of concentrated void and psionic energies. Upon realizing he could feel their psionic presences he eagerly attempted to call upon one of these slumbering 'spirits' in order to commune directly with the dead, something never before heard of among Protoss. However, Eldrun's actions proved to have been too hasty.
In his eagerness to commune with his fallen brethren, Eldrun found that the psionic presence with which he had attempted to commune appeared to be gone. But he did not feel a soul with which he could commune, instead he found that he felt younger, stronger, more powerful.
Appalled at the realization of what he had accidently done, he abbruptly severed his connection to this newfound source of power, releasing the psionice essence he had absorbed and feeling the weakness of age return to him. Eldrun left the Khalis in its chamber and returned to his home, keeping his findings secret, fearing that, should a corrupt or power-hungry being discover this power, such a being may perhaps even become immortal.
When his son, Darr, grew to be a promising successor to his work, Eldrun conferred this knowledge to him, making him promise never to use it for himself, but simply to continue in his work to find a way to give the Dark Templar access to the memories of their ancestors.
Eventually, Darr did find a way to use what his father had discovered to achieve his dream. Darr learned that by combining the energies of the Khala (which allowed the Protoss to access the minds of those alive and dead) with void energies (Which allowed Eldrun to access the essence of those long dead), the combination would allow the Protoss to commune with the psionic essences of the dead.
Eldrun lived to see his dream accomplished, and is currently still alive on Shakuras, although he is retired from duty.
By using void energies to absorb the essence of a subject, Eldrun became the first Purifier.