In reference to the IC classes on Ravenloft lore, for those who wish to deepen it, I will update here where I got the information from. Consider always, nevertheless, that much of the information has been interpreted, it may be partial, it may be from a specific point of view, and, most importantly, is meant to create more questions rather than provide full answers. So don't consider this list a major spoiler, only a reference to be used to write stories all together
First Trimester: Barovia.
Used sources:
Prisoners of the Mists own history (quests such as "The Sins of the Fathers", and several other lore provided by other DMs in time)
Domains of Dread (pp.14-19; pp.49-71, pp.75-76; pp.84-88)
Gazetteer I (pp.4-40; pp.69-74; pp.92-96)
Gazetteer IV (pp.14-19; pp.44-49; pp.112-115; pp.94-95 )
Dark Tales and Disturbing Legends (pp.110-114; p.122)
Legacy of the Blood (p.119)
Champions of Darkness (pp.78-80)
Circle of Darkness (pp.2-7)
Darklords (pp.64-67)
Van Richten's Monster Compendium 1 (pp.4-104)
Van Richten's Monster Compendium 2 (pp.85-86; pp.27-28)
Van Richten's Monster Compendium 3 (pp.96-186)
Second Trimester: Forlorn
Prisoners of the Mists own history (Quests such as "Dead Leaves on a Dirty Ground" and threads such as "Forfarian Tales and Songs"
http://www.nwnravenloft.com/forum/index.php?topic=21460.0)
Domains of Dread (pp.14-19; pp.37-38; pp.65-68)
Gazetteer I (p.18; p.20; pp.39-40; pp.42-43; pp.63-84; pp.131-134)
Gazetter IV (pp.132-134)
Gazetter V (pp. 45-96; pp.166-169)
Dark Tales and Disturbing Legends (pp.165-172, pp.110-114; p.122)
Darklords (pp.68-73)
Secrets of the Dread Realms (pp.33-35)
Van Richten's Monster Compendium 2 (pp.8-98)
Castle Forlorn Boxed Set, The Weeping Land (pp.4-65)
Castle Forlorn Boxed Set, Melancholy Meetings (pp.30-31)
Castle Forlorn Boxed Set, Eve of Sorrows (pp.27-28)
Champions of the Mists (pp.52-53)
Heroes of Light (pp.66-71)
Third Trimester: to choose with players/characters
Fourth Trimester: G'Henna
coming soon