I had PCs in "contact" with Priests Arnak several times, but I do not think that many of them are significant because they were straightforward antagonists (and combat opponents), or spies that they did not uncovered or suspected.
One PC, however, actually had a fairly lengthy discussion with a Priest Arnak when she was captured during a foray in Mur Fostisyr. The goal of the priest was not to destroy her, but to convert her. I asked the player afterwards and she agreed that having a smiling, polite and charismatic priest Arnak (the NPC never hid who he was) was much more terrifying than having a summoned demon lick her character and ask for a piece of her soul as treat. Especially since, during her escape, this particular priest watched everything and did nothing.
That priest Arnak was not a sadist or a bully. He was a scholar who was convinced that Unlife was the ultimate salvation for people because it brought peace: it was a refuge from pain, war, sorrow and the tribulations of hard life. It was the state that everyone secretely yearned for without being able to name it - darkness and warmth from before the pain of birth. He knew how to read people, how to gently point out where they hurt and how they resented the world and unwanted changes, how they wished they were unknowing and unfeeling.
The kind of person who would convince you that poisoning your pregnant wife is a blessing for both mother and child - and would not need magic to do so.
But thankfully, most priests Arnak are in it for power, personal pleasure or revenge. Otherwise Kulthea would be a dead world.
[one problem with having priests Arnak everywhere is that RM magic is crawling with investigation spells which do not require RRs, especially in Mentalism and Channeling - and PCs are notoriously suspicious. And my take is that priests Arnak know that they are vulnerable to investigation magic, so they most often act through proxies. There are ways to hide from investigation spells, but they are used only when proxies are not convenient, which is not that often]