but he perished in a skirmish near Andravida. As he had failed to pay off his Turkish mercenaries, they entered into William's service. With their support, William defeated the Greeks at Nikli and laid siege to Mistra, but he could not capture it. In response to the Byzantine offensive in the Morea, Urban IV proclaimed a crusade against Michael VIII, but he also appointed new delegates to start negotiations with him about the church union. Manfred of Sicily was willing to support the Franks against the Byzantines by force, but Pope Urban, who regarded Manfred as the principal enemy of the Papacy, rejected the offer. Instead of promoting an anti-Byzantine coalition, the papal delegates mediated a reconciliation between Michael VIII and William before the Pope died in October 1264. Angevin suzerainty Urban's successor, Pope Clement IV, granted the Kingdom of Sicily to King Louis IX's ambitious younger brother, Charles I of Anjou in 1265. Charles invaded southern Italy, and Manfred died fighting against the invaders in the Battle of Benevento on 26 February 1266. Charles took full control of Manfred's kingdom and revived his plans for an anti-Byzantine coalition. As negotiations on the church union had not yielded results, Pope Clement embraced Charles' idea. He summoned Charles, the titular Latin Emperor Baldwin II and William to Viterbo in Italy to discuss the conditions of an invasion of the Byzantine Empire by Charles. Charles and William reached an agreement after lengthy discussions. William swore fealty to Charles and promised not to "make enfeoffment which will remain in effect after" his death "beyond 14,000 of land". He consented to the marriage of his daughter, Isabella, with Charles's younger son, Philip. Although his wife was pregnant, William acknowledged Charles and Charles's descendants as his heirs even if Isabella and Philip had no children and Anna gave birth to a son, only stipulating their son's right to hold one-fifth of Achaea in fief of the Angevins. As historian Peter Lock emphasizes, William's concessions show "the straits to which the principality had been reduced by the Greek offensive". In return, Charles promised military support to recover the territory lost to the Byzantines, but the details of his assistance remained unclarified. Baldwin II confirmed the treaty and ceremoniously ratified in the presence of the Pope, 14 cardinals and further ecclesiastic and secular dignitaries in the papal chamber in Viterbo on 24 May 1267. In a separate