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He was a Turkic slave of the Seljuk sultan Malik Shah I, who had captured Antioch in 1085 and appointed Yaghi-Siyan governor around 1090. Malik Shah died in 1092, and his successor Tutush I granted Yaghi-Siyan more territory around Manbij and Turbessel. When Tutush died in 1095, his sons, Ridwan and Duqaq, fought for control of Syria, claiming Aleppo and Damascus respectively. Ridwan's claim to Aleppo was opposed by an alliance of Yaghi-Siyan, Ilghazi, and Duqaq. Yaghi-Siyan disliked Ridwan's tutor Janah ad-Dawla more than he disliked Ridwan himself, and thus allied with Duqaq instead. Ridwan and his allies attacked Yaghi-Siyan's territory, and then besieged Damascus when Duqaq and Ilghazi came to assist Antioch. In 1097 Ridwan quarrelled with Janah ad-Dawla, and Yaghi-Siyan became more amenable to an alliance. This was completed by marrying his daughter to Ridwan. The two were about to attack Shaizar when news of the crusade arrived, and all parties retreated to their own territories to prepare for the coming attacks.
Siege of Antioch 
Despite the alliance, Yaghi-Siyan was left alone to fight the crusaders with only his personal army in Antioch. To prepare for a siege, he exiled many of the Greek Orthodox and Armenian Orthodox Christians, whom he considered untrustworthy. He imprisoned the Greek Patriarch, John the Oxite, and converted the Cathedral of St. Peter into a stable. The Syrian Orthodox Christians were generally left alone, as Yaghi-Siyan considered them to be more loyal to him, as enemies of the Greeks and Armenians. Over the winter of 1097-1098, Antioch was besieged by the Crusaders, and Yaghi-Siyan and his son Shams ad-Dawla sought help from Duqaq. He frequently sent out sorties against the Christian camp, and attacked foraging parties further afield. Yaghi-Siyan knew from his informants that there were dissensions among the Christians; both Raymond IV of Toulouse and Bohemund of Taranto wanted the city for themselves. While Bohemund was away foraging on December 29, 1097, Raymond attacked but was pushed back by Yaghi-Siyan's troops. On December 30, reinforcements from Duqaq were defeated by Bohemund's foraging party, and retreated to Homs.
Yaghi-Siyan then turned to Ridwan for assistance. In February Ridwan's army was also defeated; while the crusader army was away from the city fighting Ridwan, Yaghi-Siyan marched out to attack the foot-soldiers left behind to defend the camp, but he too was pushed back when the victorious crusaders returned. In March Yaghi-Siyan ambushed the crusaders who were bringing wood and other material back from the port of St. Simeon; when the crusader camp at Antioch heard that Raymond and Bohemund had been killed, there was mass confusion, and Yaghi-Siyan attacked the rest of the army under Godfrey of Bouillon. Bohemund and Raymond soon returned however, and Yaghi-Siyan was once more pushed back into the city.
This time the governor turned to Kerbogha of Mosul for help. The crusaders knew they had to take the city before Kerbogha's reinforcements arrived. Bohemund secretly negotiated with one of Yaghi-Siyan's guards, an Armenian named Firuz, who agreed to betray the city.
On the night of June 2-June 3, 1098, the crusaders entered the city; Yaghi-Siyan fled with his bodyguard, while his son stayed behind to defend the citadel. During his escape, Yaghi-Siyan fell from his horse, and as his guards found it impossible to bring the injured governor with them, they left him on the ground and rode away without him. He was found by an Armenian who cut off his head and sent it as a gift to Bohemund.
Antioch was claimed by Bohemund and Raymond, with Raymond stationed in Yaghi-Siyan's residence and Bohemund in the citadel when it was captured from Shams ad-Dawla the next week. Their quarrel delayed the crusade for many months.
The crusaders recorded Yaghi-Siyan's name in various forms in Latin, including Acxianus, Gratianus, and Cassianus; the residence claimed by Raymond was known as the palatium Cassiani.