Wikipedia
Yaghi-Siyan (died June 3, 1098) was the governor of Antioch during the First Crusade.
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.