Dr. Cruces is a former Head of the Assassins Guild. He is first encountered in Pyramids and is a Master titled Assassin and Head Tutor. He is a well educated, soft voiced lean figure, though with 'the pitch and spin that could make (his voice) be heard through a hurricane'. During the events of Men at Arms, he develops more as a character. He is the mentor to whom the frighteningly obsessive d'Eath turns and is driven mad by the appalling power inherent in the gonne after it engineers the death of Edward d'Eath. Cruces then takes up the gonne - and d'Eath's mania for reintroducing a royal family to Ankh-Morpork, leading to a series of events that include an attempt on the Patrician's life. He is pursued by Sam Vimes and Carrot Ironfoundersson. Samuel Vimes is briefly tempted to take up the Gonne himself, but Carrot, who has a refreshingly direct approach to Cruces' blandishments, destroys the Gonne and kills Cruces without a word.
His name, 'Cruces' is the formal plural of 'Crux', the word for 'Cross' in Spanish. It not only suggests 'cross' but also 'crucify' or 'excruciating' and 'the crux of the matter'.
Upon his death, Dr Downey replaces him as the Head of the Assassins Guild, and the time-honoured profession of artistically exercising lethal prejudice for a fee continues with barely a hiccough.