It sounds like you might be talking about Roots of Evil, one of the original Ravenloft modules from 2nd Edition, and involves Azalin sending a group of PCs back in time to try to wreck the original formation of the Demiplane of Dread by interfering with the events at Sergei's wedding and stopping Strahd's Pact with "Death." It's the final part of a series of 5 modules about Hyskosa's Hexad, a prophecy of a Vistani seer who predicted the Grand Conjunction, a massive event that lead to the breakup of the Land of Mists, and each module covers one of the verses, although you can still easily run the final adventure on its own. (The Grand Conjunction is the event that rearranged the Demiplane of Dread as presented in the revised Ravenloft boxed set.)
I've owned the module for years and have read over it many times but never run it, but if I ever did I would incorporate material from the novel I, Strahd as a source of more information. Such a great book, I was actually just recommending it to a bunch of people last week. (I persuaded my non-gaming mother to read it years ago and she loved it, and last week I convinced my non-gaming aunt to find a copy to read.)
I, Strahd would give you plenty of details and feeling to use in the game, including a lot of the main guests at the wedding and Strahd's activities during the event as well as a lot of atmospheric style. The book begins in the tent encampment of Strahd's army the morning after the final assault against the Terg warlord Dorian that won the Castle that would become Ravenloft, runs through the initial years of Strahd's rule of Barovia, through the events of the wedding and his transformation into a vampire, and then goes into some followup events a few decades later that tie into the night of the wedding. It's so good. (The story as told in "The Book of Strahd" is revealed to be an edited version of the events that Strahd altered to keep people from knowing the full details of his history, and the way he is written in the novel makes Strahd an extremely cool, sympathetic, and noble yet Evil character.)
There's a great character in the novel who isn't in the Ravenloft sourcebooks, Alek Gwilym, who originally entered Strahd's command as a foreign mercenary decades before the book starts, but rose through the ranks through sheer talent to become Strahd's right hand man in the army, and finally Strahd's main security/intelligence officer as ruler of Barovia, and his only real friend (even if Strahd doesn't realize it til the end.)
If you plan on running such a game using Roots of Evil, I would highly recommend incorporating the events of I, Strahd; the events straight from the module are pretty dry.