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The Mysterious assassin is the first kingslayer; the one who tried unsuccessfully to slay King Foltest. He attempted during the ending of the original game. Geralt, who was in the process of receiving his reward for destroying Salamandra and removing Jacques de Aldersberg, stopped and killed him. There was no hint or clue as to his true identity, except that he was possibly a witcher. During the final confrontation with Letho, he talked about two other witchers, which 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 him with Triss (autopsy results) and a Temerian Recruit (from where he came), the soldier is near Crinfrid Reavers. Before entering the battle tower, after conversation with Shilard Fitz-Oesterlen, Foltest asks Geralt about him. Later, in Epilogue, when your memory recovers, you see the King of The Wild Hunt, Yennefer, Geralt, Letho and three witchers (Serrit, Auckes and maybe this assassin).