In Curse Of Strahd, The Pack uncovered secrets about Strahd's betrayal through a Page From Tome of Strahd, and Atlas Atruvian learned more of Sergei's plight via the Tome of Strahd itself.
Born nobly into the von Zarovich family to the Count Barov von Zarovich and Countess Ravenovia van Roeyen, Sergei was a well-liked, charismatic youth in the Village of Barovia. A surprise baby, he was twenty-five years younger than his older brother Strahd, and grew up in the shadow of his brother's genius. Strahd was a fearsome military mind, and was mostly away for Sergei's youth, eventually massing armies to take over his family's rightful valley and establish Barovia. From there, Strahd intended to move the von Zarovich's into a massive castle, named after his mother.
Sergei grew up idolizing Strahd, and though he lacked his brothers subtle mind for strategy, he became a renowned warrior in his own right. Devoting himself as a Paladin of the sun, Sergei worshipped the "Morninglord", the Barovian term for Thestus. He assisted Strahd in beating back the paganastic forest-folk and great houses of Western Barovia, while also fighting the creatures of darkness that resided in those lands. On one pilgrimage to Zachodgrad, he met Alizandra Markovia and established ties to the Abbey of Brilliant Worship, becoming a paladin of their church and assisting with the fort's defenses. He was gifted the Bright Blade by Saint Markovia, with a golden hilt and and glass-like edge, designed for fighting undead.
During this period, Sergei met another follower of the Morninglord, Tatyana Federovna. Though Tatyana was not of noble birth, Sergei fell in love with her instantly, and the two were engaged to be married at the newly-constructed Castle Ravenloft. Sergei did not realize, however, that his tyrannical brother had also fallen for Tatyana, and had been learning of evil forces outside of Barovia. On the day that Tatyana and Sergei were to be married, Strahd summoned the Dark Power Vampyr and invoked a pact with them; this resulted in the Wedding That Never Was.
In a fit of jealousy and desire, Strahd attacked Sergei. Sergei was overwhelmed by his brother's evil magic, and his sword's blade shattered. However, this was not enough to fight Strahd in his new form as a Vampire. He killed Sergei in cold blood, and brought his body to a tomb below the castle, never able to truly rest. The paladin's sword was lost, but would later become the "Sunsword", a blade dedicated to destroying Strahd.