Tyrone Rugen, a Count in the court of Prince Humperdinck, has a disturbing interest in the scientific study of pain and created a machine to suck the life out of a person. In other words, he is a sadist, deriving pleasure from inflicting pain on others. As he tests it on Westley, he takes careful notes on its effect on him. He is the secondary antagonist of the book and the film.
In the past, Tyrone approached Domingo Montoya, to create a sword for his unique six-fingered grip. After Domingo slaved over it for a year, Tyrone returned and offered only a tenth of the promised price. Domingo refused; Rugen slashed him through the heart. When Inigo challenged him to a duel, Tyrone bested him and left him alive, with a scar on each cheek as a reminder. Inigo spent the next 20 years planning his revenge and searching for Rugen, identifying him by the six fingers on his right hand.
In the end, Inigo duels with Rugen while repeating his well-practiced phrase during the duel. At the end, Rugen bribes Inigo to let him live with the thought of money and power. Terrified for his life, Tyrone agrees to give him whatever he wants. Inigo stabs him through the chest, saying "I want my father back, you son of a bitch".
It is interesting to note that the five wounds suffered by Count Rugen at the hands of Inigo in the climactic duel — two slashes to the face, a stab in his upper right arm, a stab to his left shoulder, and the killing thrust through the abdomen — exactly correspond to the five wounds Rugen had earlier inflicted on Montoya (the two slashes to the face when he was the eleven-year-old "Spanish brat", a wound to his stomach caused by a knife thrown by Rugen, and stabs in his shoulder and upper arm after Rugen attempted to end his repeating of his phrase).
It is possible that Rugen was born with ulnar polydactyly, giving him his memorable six fingers.