Prad Vidal was the Duke of Elacca, a sub-continent on the Ecazi Homeworld, and a vassal to Archduke Armand Ecaz. He was responsible for the murders of the Ecazi heiress, Ilesa Ecaz and the Swordmaster Rivvy Dinari during the wedding ceremony of Ilesa and Duke Leto Atreides I on the planet Caladan.
Duke Vidal was the leader of House Ecaz, an ally of the Harkonnens and an avowed enemy to the "Old Duke" Paulus Atreides. The ruler of planet Ecaz however is the Archduke Armand, who is also an ally of House Atreides. The Archduke even tried to arrange a marriage between his eldest daughter Sanya Ecaz and Duke Leto.
Duke Vidal participated in the attempt made by Viscount Hundro Moritani, the great enemy of House Ecaz, to assassinate the Archduke and his daughter, as well as some members of the Atreides' household, during the wedding of Ilesa and Duke Leto Atreides, on Castle Caladan. Duke Vidal cleverly disguised hexagonal blades on the sides of potted plants at the ceremony. As the Wedding March began to play, the sharply bladed discs began circling through the air, trying to strike their intended Ecazian targets, and Swordmasters Rivvy Dinari and Whitmore Bludd defended the Archduke and his daughter, respectively.
However, one circling disc sliced Ilesa's cartoid artery, due to Bludd's failure, and she bled to death, while her father lose and arm and barely survived due to the sacriface of Dinari, who protected his master with his own body. Moments after the tragedy, a message of Viscount Hundro Moritani rang on the hall, starting the War of Assassins of 10,187 AG.
Immediately after the assassination attempt, which caused the death of the last Ecazi heiress, another attempt to kill the Archduke took place on a tower of Castle Caladan, where the old Ecazi ruler was resting after having lost an arm. The assassins were defeated by Duncan Idaho, while other assassins sent to kill Paul Atreides and Bronso Vernius were defeated by Count Rhombur, father of Bronso.
It was then that Archduke Armand decided to leave Caladan for Ecaz, in order to assemble his forces to attack Grumman, the homeworld of House Moritani. Duke Leto assembled his navy and army, and marched to Ecaz escorting the man that would have been his father-in-law, so that they could join their forces and defeat the Viscount. As soon as the Atreides army arrived on the Ecaz' capital city, the people started to flee in panic, thinking that the Atreides' forces were invading the planet. The greatest confusion, however, took place on the Archduke's Palace, where a lot of ships started fleeing in direction to Elacca, and when Swordmaster Bludd asked why the Ecazi gave such a welcome to their sovereign, a guard answered through the communications channel that Duke Prad Vidal had announced that the Archduke and his daughter have been killed on Caladan, as well as many members of the Atreides household, and that he was the successor of House Ecaz due to the circumstances.
When the Archduke arrived in his Palace, Duke Vidal had already escaped from there, running away to his estates at Elacca, where he had organized the defense of his dominions. Duke Leto realized that if his army was delayed by Vidal's Elaccan troops, Viscount Moritani would have more opportunities to gather more troops in order to defend Grumman, so he proposed Armand to end Vidal's threat in the most easy way: by assassinating him. Whitmore Bludd asked permission to his lord to participate in the assassination of Duke Vidal, however, the Archduke (disappointed of Bludd's failure to protect his daughter), ordered him to stay in the Palace, in order to gather more troops to the Ecazi army.
Duke Leto and his loyal trobadour warrior Gurney Halleck, disguised as Elaccan guards, entered in the Wooden fortress of Prad Vidal in Elacca, and as soon as they found him, Duke Leto pronounced the ritual words that start a War of Assassins, and beheaded Duke Vidal with a sharped blade similar to that ones employed on his wedding day.