The full story of the being known as Priest may never be known, but from various accounts an attempt to understand his history can be made.
Hundreds of years ago Priest came into conflict with Lord Malvolio of the Green Flame. Malvolio wielded the power ring of his father, whom he had murdered. Priest had been chosen by the Guardians of the Universe as the rightful successor to the deceased Green Lantern of Selsia V and was assigned to eliminate Lord Malvolio.
Instead, Priest disobeyed orders and somehow tricked Malvolio, trapping his foe in a starless void. Malvolio would watch Priest, plotting his captor's downfall.
For his refusal to follow orders, the Guardians stripped Priest of his power ring and banished him to an asteroid in Space Sector 1634.
As Priest later claimed to Hal Jordan of Earth, he discovered a way to tap into the Guardian power without using the power ring or battery. For a time, he claimed he was the greatest of Green Lanterns. He said he left the Green Lantern Corps to follow a more "spiritual" path. Priest had achieved a depth of understanding in the Green Lantern Corps that few, if any, could conceive of.
Priest lured Hal Jordan across the stars by destroying Jordan's power battery. At this time, the Earth's Green Lantern was one of the few emerald warriors still active after the execution of Sinestro and the collapse of the Central Power Battery. Jordan found great difficulty in the use of his power ring. Normally, when travelling to other sectors, the ring would draw on the Central Power Battery on Oa and reprogram itself.
When Jordan reached the end of his journey he came upon the former Green Lantern. Confronting the alien, Jordan's power beam was grabbed and the Green Lantern of Earth found himself being pulled through space by the red skinned alien. The ring would not obey Hal's will.
On a small planetoid, the alien told Jordan he had been brought to halt a war between two worlds, a war that threatened the safety of the galaxy. The alien retrained Jordan in the use of the power ring and attempted to explain his past to the Green Lantern of Earth. Jordan dubbed the alien "Priest", an incorrect term, but as close to accurate as was available in human language.
Priest told Jordan that Space Sector 1634 was once patrolled by a noble Green Lantern, worthy of respect and acclaim. This Green Lantern took a far more active role in policing this part of the galaxy than Hal Jordan was accustomed to. He was idolized, almost deified. His benevolent influence kept the sector at peace for generations. Now, Jordan would stand in his place.
Although Jordan agreed to the task, he wished to recharge his power ring. He believed that time moved slower in Priest's sector, and that is why he still had power.
Priest dismissed Jordan's concerns. He told the Green Lantern he did not need the battery or even the ring. He could tap into the lantern power source directly.
Jordan was infuriated by Priest's answer. He believed the alien would not be able to put the power battery back together and that Jordan would soon be doomed, without power.
Priest gave Jordan his battery, intact but missing the power essence that would focus the energy. The Green Lantern of Earth would need to go to sub-sector 82, to find the essence within a buoy stationed there. The trip would take about an hour.
It was several hours later when Jordan reached the buoy. Inside he found a green glowing sphere. He was surprised by attacking dreadnaughts. Jordan was warned that he stood in a war zone and wore the symbol of their god of Selsia V. They believed their god would never side with their enemies. The aliens left quickly, and Hal quickly unscrewed the top of his lantern to replace the power essence. He was too late and blacked out.
He awoke to the sight of Priest. The alien old Jordan that since he believed he was charging his ring, Jordan tapped directly into the source of his ring's power. This was not believed by Jordan, despite Priest's urging.
Jordan soon arrived in the midst of the warring parties. He used a show of force to convince the leaders of both sides, Janel and Hedron, to sign a peace treaty.
The Green Lantern stopped by Priest to report on the outcome and to offer his thanks and returned to Earth. Though Jordan thought he had been successful, he had only caused the warring parties to band together against him.
Jordan would return to Earth, the next time he returned to Priest's sector was in pursuit of an alien shrine that had caused major destruction in Los Angeles. He was plucked from space by Lord Malvolio, who sought Jordan's aid in destroying Priest and escaping his prison. Although Jordan did escape from Lord Malvolio, it was with Malvolio's power ring instead of his own. Unknown to Jordan, this is what Malvolio had planned.
The leaders of Selsia IV and V had combined their technology to create the Seeker. This was a mechanical juggernaut that sought out Green Lantern, destroying everything in its way.
The apparant return of a Guardian and a Zamaron to Oa marked another chapter in the lives of Jordan and Priest. Hal Jordan and Chip, a friend from Jordan's workplace, were summoned to Oa. The Guardian ordered the death of Priest for the greter good of the universe. If Jordan refused, his friend Chip would be killed.
Jordan sought out Priest who encouraged the emerald gladiator to follow the Guardians wishes. Priest argued Jordan would have to kill him for the greater good of the universe.
The Green Lantern was occupied at that moment, fighting the Seeker construct. Following the advice of Priest, Jordan gave up his power ring which the Seeker had been tracking. Deprived of a target, the Seeker shut itself off. It was with this victory that Jordan made his decision regarding Priest.
Returning to the Guardian, Jordan would not kill Priest. Though Chip's life had been threatened, Jordan considered Priest as benefitting the greater good by teaching him how to be a more effective Green Lantern. It was at this point the lesson was over. The Guardian, Zamaron, and Oa faded away, all an illusion created by Priest.
Priest had staged the Guardian and Zamaron's return as a way to test Jordan for Malvolio's influence. Priest, Jordan, and Chip visited the real Oa where Priest offered his actions as pennance for failing the Guardians so many years before. Though Priest had indeed made Jordan a better Green Lantern, this failed to impress Appa Ali Apsa.
Appa Ali Apsa had changed, he now took to riding a Harley Davison motorcycle across the Oan sands. The puzzling actions concerned Priest, who had never seen this behavior in a Guardian. He decided to remain on Oa with Appa, trying to help the lone Guardian.
The isolation from his brother Guardians caused Appa Ali Apsa to slip into madness. Appa wanted Priest to join in a mind-union. The former Green Lantern refused, fearing he would not be able to get out again. Appa wanted to know why he would refuse when he could be joined to the Guardian forever. Priest's answer was he'd thought the Guardian had gone insane. Priest was killed soon after.
The insane Guardian went to the Oan sciencecells to find company. He wanted to join with the prisoners but began killing them one by one out of spite. When he came upon Goldface, the quick thinking criminal made a deal. He promised if Appa Ali Apsa would send him to Earth, he would recruit others to keep him company. Many of the other convicts had the same idea and were thus spared. On Earth, Goldface gathered his rivals to send to Oa. He never heard from the Guardian again.
Arriving on Oa, John Stewart found the body of Priest. Appa Ali Apsa told him of Priest's story. Now Stewart would be joined with the insane Guardian... Priest was one of the many memories the Guardian left with Stewart.
Priest is memorialized in the Crypts of the Green Lantern Corps on Oa.