He was one of the first witcherGeralt of Rivia met upon his arrival in Murky Waters. He told the witcher of Berengar's presence in the area and also of the strife brewing between the vodyanoy and local humans.
Although the friendly priest was often driven away from his post at the foot of the altar by Dagonworshippers, Geralt dispatched them at that time so that the friendly priest could return.
In order for him to really return, the main quest The Heat of the Day must be advanced up to the point of Alina's and Celina's transformation as well. Otherwise, the enemy vodyanoy will keep respawning.