It is interesting that no one mentions the Souragnian worship of Loas, aka spirits, in this thread.
The most suitable class would be Voodan, but at NWN we don't have it. So the closer of it would be cleric or druid (for some less harsh Loas).
In some way to worship all the Loas (or at least the darkest group of them) would be close to be worshiping the Legion of the Night and their wicked sources of power. The main Loa is the Lord of the Dead, who might claim the body of the unburied dead in four days to put at his service. And if that isn't enough for you, know that there is freedom to come out with another main source of worship as the Loas are as many as many are the existing things.
The Gods were silenced in Ravenloft, but not the spirits, who might act as their messengers. So a cleric that worships these spirits will have more contact with their source of power than most of the priests that lurk through our setting.
Also, it is important to mention the relation of the Loas with the Ezra cult. In Souragne people would mix up both beliefs. As Ezra is the religion of the most powerful and rich, it is imposed over the population. The open worship of the Loas is usually practiced mainly by the masse. Although, when everything fails and despair comes it doesn't matter how wealthier you are, in Souragne you will be looking for the aid of the Voodans. This kind of thing lets out open door to twists of the Ezra Church beliefs and adaptations accordingly to the suitable background you seek.