Others have commented the assassin is a time traveller. Time travel has nothing to do with it! Affect an unrelated thing in such a way that the knock on effect leads to the death of the intended victim.
ASTROMANCY [noun] divination by means of the stars. Etymology: from Mediaeval Latin astromantīa < Greek astromanteía, from astḗr, “star” + manteía, “divination”.[Pete Mohrbacher - Kokabiel, Angel of the Stars]