The Mysterious assassin was the first known member of the Kingslayers and was unsuccessful in his attempt to kill King Foltest. Geralt, who was in the process of receiving his reward for destroying Salamandra and removing Jacques de Aldersberg, was in the castle when the assassin made his move. An intense sword fight played out between the two before Geralt killed the would-be assassin. His identity was never revealed but it is believed that he was a witcher due to his eyes.
During the final confrontation with Letho, he talks about two other witchers, who may or may not be alive and on the Path, but he never mentioned this assassin in particular.
In the prologue, Geralt can talk about this assassin and the autopsy results with Triss and where he came from with a Temerian recruit near the Crinfrid Reavers. Before entering the battle tower, after a conversation with Shilard Fitz-Oesterlen, Foltest asks Geralt about his attempted killer.