Ostrit was a courtier in the court of King Foltest who loved the king's sister, Adda of Temeria. Angered by the incestuous relationship between the king and his sister, and the pregnancy it led to, Ostrit cursed them.
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.
He appears in both, first Polish comic showing short story illustrated by Bogusław Polch under name "Geralt" and is also shown in the second issue of "The Witcher: Curse of Crows" when Geralt retells Ciri backstory of his first striga contract.
In the TV series, Ostrit is portrayed by Andrzej Słabiak.
In the game, the cinematic intro sequence features Geralt using Ostrit as bait for the striga. 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.