Guillaume Arnaud

William Arnaud or Guillaume Arnaud (died 28 May 1242) was a Dominican inquisitor and martyr.

William was a native of Montpellier. In 1234, Pope Gregory IX named him inquisitor in the dioceses of Agen, Albi, Carcassonne and Toulouse. He was also active with Pierre Seilan in the diocese of Cahors. He was learned in canon law and gained a reputation for zealousness in his inquisitorial duties. He raised enough opposition that he was banished from Toulouse in October 1235 and only allowed back in March 1236 after papal intercession with the count. The Chronicle of is an important source for William's inquests, since Pelhisson had access to its now lost records. He appears to have been the most active inquisitor in the region in the 1230s.

On 28 May 1242, William and eleven others were massacred at Avignonet. Their murder was arranged by the bailiff, Raymond of Alfaro, perhaps with the tacit agreement of Count Raymond VII of Toulouse. The murderers themselves came from the castle of Montségur. None of the murderers were ever punished. The Cathars celebrated William's death, even composing songs in Occitan about it, according to an inquisitorial deposition from 1244. The Cathar leader expressed a desire to drink wine from William's skull, had it not been crushed to pieces. The fictionalized but historically based account in the Novas de l'heretje is probably referring to William under the name "Huc Arnaut":

Catholics reported miracles following the deaths at Avignonet, but it was not until 6 September 1866 that they were beatified by Pope Pius IX. William is listed in the revised Roman Martyrology of 2004, but not in the General Roman Calendar.