Around 30-40 years ago, he led a charge to defend the Shayziens' Wall from a lizardman attack. At the start of the battle, an arrow impaled his eye, knocking him out and resulting in him missing most of the battle. By the time he woke up, he found himself in lizardman territory and was worried for his life.
Contrary to his beliefs, the lizardmen treated him well. He hoped for his fellow Shayzien to rescue him when they did not find his body among the corpses, but they never came. When he regained his strength, the lizardmen let him leave, so he went back to the wall. Unfortunately, his time with the lizardmen had changed his appearance, so when he arrived at the wall, the Shayzien believed him to be a lizardman and fired on him.
He returned to the settlement and found a shrine, where he lives in to this day.
In addition, after using The fisher's flute on Kharedst's memoirs, it can be used on the Swamp Priest, which reveals that he knew Rose, the daughter of King Kharedst IV, the last king of Great Kourend.
Swamp Priest: Yessss, adventurer?
Player: You smell funky.
Swamp Priest: As do you, adventurer.
Player: How come you live with the Lizardmen?
Swamp Priest: They are my sssaviours, I owe them my life.
Player: What did they save you from? Is it to do with your eye?
Swamp Priest: Why, yesss it isss... Many yearsss ago now, 30? 40? I've lost count.
Swamp Priest: I wasss a Captain of the Shayzien Watch, Captain Reynard Koil, ssstationed up on the Shayziens' wall.
Swamp Priest: We were told to attack the Lizardmen, for they were apparently attacking usss. I led the charge.
Player: Apparently? Were they attacking?
Swamp Priest: Alasss, I took an arrow to the eye and missed the battle. I woke up here, worried for my life.
Player: The Lizardmen took you prisoner?
Swamp Priest: At first, that isss what I thought, but they tended my woundsss and fed me well.
Swamp Priest: I assumed my people would come for me once they sssaw my body wasn't amongssst the dead, but they never did.
Player: Would the Lizardmen let you leave?
Swamp Priest: Oh yesss, once my ssstrength returned, I went and ssspoke to the Shayzien Watch.
Swamp Priest: They thought I wasss a Lizardman and fired arrowsss at me, ssso I returned here.
Player: What is that you're wearing?
Swamp Priest: I found these in thisss shrine, I guesss they belonged to sssome long dead priest.
Player: What would a priest be doing in this gross swamp?
Swamp Priest: This grossss ssswamp is my home, you sssure do ask a lot of questionsss, adventurer.
Player: What is the symbol on the shrine?
Swamp Priest: You do not recognize it, adventurer? It would seem hissstory is written by the winnersss, for that isss the sssymbol of the Great Lord Xeric.
Swamp Priest: Or at leassst, that isss what the Lizardmen tell me...