William Longsword (1140s – June 1177) was a member of the House of Montferrat from northern Italy who became the count of Jaffa and Ascalon in the Kingdom of Jerusalem. As a cousin of Emperor Frederick Barbarossa and King Louis VII of France, William was chosen by the High Court of Jerusalem to marry Sibylla, the heir presumptive to the kingdom. William may have expected to rule in the name of her young and sick brother, King Baldwin IV. He arrived in the kingdom in late 1176, but the nobility were no longer well-disposed to the proposed match. William nevertheless married Sibylla and as her husband, received Jaffa and Ascalon. Because Baldwin IV had leprosy, it seemed likely that William would eventually succeed him as king. William died of an illness within months of his marriage, however, leaving Sibylla pregnant with their son, Baldwin V. Rumors spread in Europe that William had been poisoned.
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Background
William was born in the 1140s. Hailing from Piedmont, he was the eldest son of Marquis William V of Montferrat, a vassal of the Holy Roman Emperor Frederick Barbarossa. William Longsword's mother, Judith, was the daughter of the Babenberger margrave of Austria, Leopold III. Through his grandmothers, Agnes of Waiblingen and Gisela of Burgundy, William was the first cousin of both Emperor Frederick and King Louis VII of France, a relationship emphasized by Archbishop William of Tyre. The House of Montferrat loyally supported the emperor in his long conflict with the Lombard League, who were allied with the papacy and King William II of Sicily.