When the carcass of Kos washed ashore, the residents of Fishing Hamlet were turned into fish-hybrids. The priest was one of those who turned at that time. The transformation lined eyes into their brains and they started worshipping the beached Great One as their deity ever since.
But alas, that event caught the attention of the scholars of Byrgenwerth.
The priest is one of a few survivors of a hunter raid organized by the scholars of Byrgenwerth long before they discovered the labyrinth beneath Yharnam.
The priest is now trapped inside the Hunter's Nightmare, wandering and muttering a baneful chant to "Mother Kos" for the doom of the hunters who commited the genocide.
"Byrgenwerth... Byrgenwerth... Blasphemous murderers... Blood-crazed fiends... Atonement for the wretches...By the wrath of Mother Kos... Mercy for the poor, wizened child... Mercy, oh please..."
"Lay the curse of blood upon them, and their children, and their children's children, for evermore. Each wretched birth will plunge each child into a lifetime of misery. Mercy, for the poor, wizened child...Let the pungence of Kos cling, like a mother's devotion..."
"Byrgenwerth... Byrgenwerth... Blasphemous murderers... Blood-crazed fiends... Atonement for the wretches... By the wrath of Mother Kos...Mercy for the poor, wizened child... Mercy, of please..."
"Curse here, curse there. A curse for he, and she, why care. A bottomless curse, a bottomless sea, source of all greatness, all things that be. Listen for the baneful chants. Weep with them, as one in trance. And weep with us, oh, weep with us..."
"Ah, sweet child of Kos returned to the ocean... A bottomless curse, a bottomless sea... Accepting of all that there is... And can be..."
It is seemingly impossible to kill this NPC as he has no way to lose health. Even throwing him down the chasm on the sides of the path that leads to the Fishing Hamlet lamp will still result in him surviving the fall and walk in the floors of said chasm.
Gives Accursed Brew if spoken to twice with the Milkweed rune equipped.
The priest does not acknowledge the presence of The Hunter, possibly because The Hunter is not imprisoned here like the blood-drunk hunters.