Black Sun Rising is the first book of the Coldfire Trilogy by C.S. Friedman. Priest Damien Vryce teams up with The Hunter, an evil he has sworn to destroy a greater threat. It’s a buddy-cops on a journey story. They each affect the other and at the journey’s end, both are changed.

Eventually Damien asks the Hunter what he is, to be able to better support / predict the Hunter when they finally confront their enemy. The Hunter tells Damien that his power manifests by inspiring fear in those around him.

“For you I’ve become the most subtle creature of all: a civilized evil, genteel and seductive. An evil you endure because you need its service—even though that very endurance plucks loose the underpinnings of your morality. An evil that causes you to question the very definitions of your identity, that blurs the line between dark and light until you’re no longer certain which is which, or how the two are divided. —That’s want you fear most of all, priest. Waking up one morning and no longer knowing who or what you are.”

I would have read this for the first time in 2000/2001 at the recommendation of a friend. This description of evil has stuck with me ever since.