[1] He was the grandson of Beatrice, youngest daughter of Marianus IV of Arborea and Timbra de Rocabertí, and Aimery VI of Narbonne (married 1363).
When Marianus V, the youngest son of Beatrice' elder sister Eleanor, died in 1407, Arborea experienced a succession crisis.
The right battle was broken into two part, the first was being chased to Sanluri and eventually was routed there, and the other one was forced by William to take refuge in the castle of Monreale, in the nearby village of Sardara, and continued the resistant.
William returned to France to seek aid, leaving Leonardo Cubello in charge in his absence.
With the chances of victory turned out to be grim, William reluctantly surrendered and sold Arborea to Alfonso V on 17 August 1420 for 100,000 gold florins.