William of Montferrat was born in Montferrat in northern Italy, a Marquesate that was led by his father, William IV of Montferrat. William was born a nephew of Pope Callixtus II, the step-brother of Louis VII of France through his half-sister Adelasia of Moriana, and was the half-brother of Amadeus III of Savoy. He became Marquess upon his father's death in 1136, and married Judith of Babenburg, who was daughter to Leopold III of Austria. William's children would include King Conrad of Jerusalem, who was a rival for the throne of Jerusalem with the King of England.
He participated in the Third Crusade and was a part of Richard I of England's army, although the two were alienated following William's execution of Richard's 3,000 Saracen prisoners whom he intended to ransom as a message to enemies of the Knights Templar, whom he was affiliated with. He was left as regent of Acre as the king was away fighting, and often retreated to his Citadel when challenged by Richard. William plotted with the Templar Order to hand over Acre to them, although some believed that he was trying to give the city to his son Conrad, Richard's rival.
Altair headed to the citadel, finding Richard leaving the castle on horseback with his horsemen. He argued with William because he massacred prisoners, which would increase the resolve of the Saracens. Richard insulted William, and he left his citadel. William was left there to argue with his own men, and told his officers to gather his soldiers so that he could review them.
He followed him to his citadel and found a group of soldiers. After a duel, he tripped Montferrat and stabbed him in the neck with both of his hidden blades. He found out from a dying William that he would not give Acre to his son Conrad of Montferrat nor kill Richard the Lionheart, but leave Acre's government to the people. Altair called him cruel and said that his acts could not continue, and William told them that the Assassins did not free cities, but damned them.