I thought it was the death of the unit, but it would seem that you get one "smite" for each instance the priest is damaged. So, if he/she is hit twice, you get two bolts even if the priest is still alive. At least that's what I've concluded from casually watching battles, i.e. without doing extensive in-depth testing.
I already thought of giving Coral queens cold resistance (not immunity) items and regeneration rings, and trying it out with BoW. A bit costly and risky, but could be fun. Sort of like holy berserk in a way - the holy protector gets angry at the sight of priest's blood, regardless of how it was drawn.
