Jacques Saunière, an author and curator at the Louvre, is the Grand Master of Priory of Sion. He is murdered right before the novel opens. As Grand Master, he knows important secrets about the Holy Grail, such as its location. He remains an important presence in the novel despite being dead, because he leaves a trail of clues for Langdon to follow. The list of past Grand Masters of the priory show they have all been important people and include Leonardo da Vinci.
The Senechaux are the three men in the Priory of Sion who rank just below the Grand Master and are a part of his inner circle. They too know the location of the Holy Grail. They are trained to lie about what they know to keep the secrets of the grail safe. They are being murdered in the novel by Silas, who works for Opus Dei.