The Wedding That Never Was was the single most important event in the history of the Zarovich Valley, and occurred during the union of Sergei von Zarovich and Tatyana Federovna. The two were invited to Castle Ravenloft at the behest of the Count Strahd von Zarovich himself, and had traveled from the Abbey of Brilliant Worship to the castle to live with him permanently. There, they planned to age as a family, but the Count's dark wants completely turned the fate of the valley.
The brilliant and ambitious Strahd had spent his youth re-taking the valley. Historically owned by the von Zarovich family, he renamed it to "Barovia" in honor of his late father Barov von Zarovich. Having lost both his parents during the campaign, he sought for further knowledge against death, and found it by happenstance in the Amber Temple. He had charmed his way past the temple's guardian Neferon and made contact with an ancient sarcophagus, though had chosen at the time not to invoke the power that he knew lay within.
Strahd had long harbored desire for the young Tatyana, but her love was only for his brother Sergei. On the day of their wedding, he was overcome by a secret hatred of both his brother and his own mortality. Strahd made a pact with an entity he knew as "Death", truthfully a Dark Power named Vampyr. The being completely overtook Strahd, and the Count consummated the deal with the murder of his brother, Sergei. They battled, but Strahd's newfound strength allowed him to easily defeat his brother, shattering his Bright Blade and piercing him through the heart.
Strahd professed his love to Tatyana, but she rejected him. As he chased her around the high castle gardens, she jumped from the walls of Castle Ravenloft to deny his dark desires, disappearing into the mists below. In sorrow and rage, he went on a rampage, killing High Priest Ciril Romulich, his staff, his army, and all who attended the event. He was pierced by the arrows of the guards, but Strahd did not die. Now a Vampire, he began to rule over Barovia in a much different fashion. By using a Portal installed in the castle's catacombs, he traveled to the Amber Temple, defeated Neferon, and destroyed Vampyr's sarcophagus, releasing the Dark Power into the world.
Vampyr ousted Barovia from the Prime Material and into a mist-ridden demiplane of the Umbral Realms, only accessible at Strahd's whims. Details about the event and everything subsequent became scarce, as Strahd worked to keep the denizens of Barovia uneducated and terrified. However, at some point, he subsumed the power of the Ladies Three and drove them from the plane. In concert with Vampyr's abilities, he completely controlled the land itself. Few knew the details of the wedding, as Strahd tied up all loose ends in the coming years, save those who found it recorded in the Tome of Strahd.
For hundreds of years after the Wedding that Never Was, Barovia's history warped around the Count until the Slaying of Strahd.