Indeed, both the mother and the girl-child died during the labour, and they were buried. Several years later, Ostrit's curse bloomed and the girl rose as a striga, haunting the streets of Old Vizima. Ostrit was killed by the monster when Geralt of Rivia used him as a bait to draw the striga out of its crypt.
In the graphic novels EditHe appears in both, first Polish comic showing short story illustrated by Bogusław Polch under name "Geralt" and than is also shown with second issue of "The Witcher: Curse of Crows" when Geralt retells Ciri backstory of his first striga contract. He describes him as following:
"They (crows) remind me a bit of Ostrit, Foltest's courtier. A jabberer like few others. He and Velerad told me both Queen Adda and her daughter had been thought dead. Dead and buried. (...)The man had been in love with Adda. One of those fierce, grinding loves that eat you up inside. And that man, chubby little Ostrit, had to stand aside and watch as the woman he loved was sullied by the king, her brother, who even stuck her with child. Can't say I envy Ostrit. Must've devastated. Though I also can't condone what he did next."
- - Geralt to Ciri
In the TV series, Ostrit is portrayed by Andrzej Słabiak.
In the game, the cinematic intro sequence features that ordeal. In the last chapters of the game, we learn more about the man when Geralt finds Ostrit's journal who spells out who was responsible for the original curse.