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Against the Byzantines

In his last enterprise, Guiscard mounted an attack on the Byzantine Empire, taking up the cause of Raiktor, a monk pretending to be Michael VII, who had been deposed in 1078. Robert believed he had the right to rule in the empire, because Constantine, Michael VII's son, had been proposed to his daughter Olympias. It was then clear that Raiktor was not in fact Michael, but this still was an opportunity Guiscard could not resist.

He sailed with an army of Norman and Lombard conscripts from all parts of his territories (1,300 knights according to Geoffrey Malaterra; up to 10,000 troops according to Orderic Vitalis) against the empire in May 1081. He defeated Emperor Alexius I Comnenus at the Battle of Dyrrhachium in October 1081, and by February 1082 he had occupied Corfu and Durazzo. Guiscard was ally to kingdom of Duklja and Constantine Bodin, to whom he married his vassal's daughter Jaquinta of Bari in 1081. In 1083, Guiscard destroyed the town of Cannae, leaving only the cathedral and bishop's residence. He was recalled to the aid of Gregory VII, however, who was besieged in Castel Sant'Angelo by Henry IV, in June 1083. This time it was the abbot of Saint Benignus of Dijon, Jarenton, who mediated between Guiscard and the pope.

In May 1084, Guiscard marched north with 36,000 men, entered Rome, and forced Henry to retreat. A rebellion, or seditious tumult (émeute), of the citizens led to a three-day sack of the city, after which Guiscard escorted the pope to Rome. Guiscard's son Bohemund, for a time master of Thessaly, had now lost the Byzantine conquests. Guiscard returned with 150 ships to restore them, and he occupied Corfu and Kefalonia with the help of Ragusa and the Dalmatian cities (which were under the rule of Demetrius Zvonimir of Croatia). On 17 July 1085, Guiscard died of fever in Kefalonia, at Atheras, north of Lixouri, along with 500 knights. He was buried in the Hauteville family mausoleum of the Abbey of the Santissima Trinità at Venosa. The town of Fiscardo on Kefalonia is named after him. On his tomb, there are four Latin verses, with the last one reading "Hic terror mundi Guiscardus," which translates to "Here lies Guiscard, the terror of the world."

At his death Guiscard was duke of Apulia and Calabria and Lord of Sicily. His successes had been due not only to his great qualities but to the "entente" with the Papal See. He created and enforced a strong ducal power, which was nevertheless met by many baronial revolts, including one in 1078, when he demanded from the Apulian vassals an "aid" on the betrothal of his daughter. In conquering such wide territories he had little time to organize them internally. In the history of the Norman kingdom of Italy, Guiscard remains essentially the hero and founder, though his career ended in "something of a dead end," while his nephew Roger II was the statesman and organizer.