The person you are thinking of is Bohemond of Taranto, who famously refused to hand over Antioch to the Byzantine Empire after he played a major role in its taking.

In April 1097, before the crusading leaders set off towards the Near East, they met with Alexios Kommenos, Emperor of the Byzantine Empire. All the leaders of the crusade, with the exception of Raymond of Toulouse, very grudgingly swore an oath of fealty and paid homage to the Emperor in return for naval and military support, provisions and secure lines of communication, the things the crusaders needed the most for a campaign in the Holy Land. According to the Alexiad, the main Byzantine source on the First Crusade which was written by the daughter of Alexios Kommenos, the tenor of the oath was that:

All towns, districts and fortresses which he might in the future subdue and which had previously belonged to the empire of the Romans would be handed over to the senior officer sent by the emperor for this purpose.

To ensure the Crusaders did actually do so, Alexios sent a Byzantine general, Tatikios, to accompany the Crusaders as the ‘senior officer’. Initially, both sides respected the agreement, with Alexios providing large quantities of food and materials for the siege of Nicaea in 1097, as well as a naval force to prevent supplies from reaching the city through Lake Ascanius. Although the city surrendered through secret negotiations between the defenders and the Byzantines at the expense of the crusaders, most of our Latin sources, from Stephen of Blois to Anselm of Ribemont, agree that Alexios was able to placate the knights and princes with generous loot from the city.

Unfortunately for Alexios, this cooperation would not last at the siege of Antioch, which lasted from 21th October 1097 to 2nd June 1098. From December 1097 onwards, the crusaders were struggling to find food in the nearby country, with foraging parties harassed by Turkish horsemen. Morale plummeted, and as the situation did not improve, Tatikios departed the siege in early February 1098 allegedly to seek imperial aid. Although many Latin sources accused Tatikios of cowardice, these accusations were most likely written in hindsight as the Byzantine general had left before news of the approach of a major enemy relief force reached the crusaders. According to John France, it is most likely Tatikios’ departure to request imperial aid was accepted by the crusader leaders at that time. Faced with the prospect of annihilation when a strong Turkish relief force led by Ridwan of Aleppo approached, the crusaders made an unprecedented decision to appoint a singular military commander - Bohemond of Taranto. On 9th February, Bohemond led the crusaders to an impressive victory over superior forces. This engagement greatly enhanced the military reputation of Bohemond and allowed the crusaders to tighten the screw on the defenders of Antioch.

At this time, Bohemond had apparently convinced an Armenian commander defending Antioch named Firuz to defect. Bohemond than proposed a competitive siege with other crusader leaders, with the city going to the victor. This was rejected by the other leaders as out of hand, but news of the approach of another strong relief force managed to convince the leaders that Bohemond could have the rights to the city if he facilitated its capture, on the condition that:

if the emperor come to our aid and fulfil all his obligations which he promised and vowed, we will return the city to him as it is right to do.

On 2nd June 1098, Firuz betrayed his section of the walls to Bohemond and after a long, hard siege, the crusaders finally captured the city of Antioch other than its citadel. But now it was the crusaders besieged inside the city as the Turkish relief force under Kerbogha settled down for a reverse siege. In dire circumstances, the crusaders once again turned to Bohemond to lead a breakout. In a frankly miraculous victory, the crusaders, reduced to essentially an infantry army due to a dearth in horses, triumphed over a fresh, well-fed and mobile force twice their size.

Immediately after their victory, the crusaders come into conflict over the control of the city. The citadel of Antioch had surrendered to Bohemond, who then succeeded in convincing all the leaders except Raymond of Toulouse to surrender to him the gates and towers they held along the city wall. Raymond continued to argue that the crusaders should abide by the agreement with Alexios, but he was evidently not as popular as Bohemond. Although modern scholarship has argued that Alexios had never intended to fully commit to the crusading enterprise, preferring to sit back and see if the crusaders would succeed, the crusaders were outraged when news reached them in the summer of 1098 of the turning back of a Byzantine relief force, which had apparently received false news from deserters that the crusaders had failed in their siege of Antioch. The acceptance of the other crusading leaders of Bohemond’s claim on Antioch reflected the less than favourable reputation of the Byzantine Empire in the Crusader camp.

Alexios’ envoys who finally arrived at Antioch in spring 1098, found a city firmly under Bohemond’s control. When they caught up to the rest of the crusaders who had continued on towards Jerusalem and complained bitterly that the crusaders had broken their oaths by refusing to hand over Antioch, the envoys received little sympathy. To the crusaders, Alexios had broken the agreement by not following immediately with a large army and by omitting to send provisions. This disagreement between the Byzantines and the crusaders which stemmed from perceived betrayals from both sides, led to the establishment of the Principality of Antioch, ruled by Bohemond and his descendants until the thirteenth century.

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Sources:

Shepard, Jonathan, 'When Greek meets Greek: Alexius Comnenus and Bohemond in 1097-98', Byzantine and Modern Greek Studies 12 (1988), 185-278.

Murray, Alan V., 'The Enemy Within: Bohemond, Byzantium and the Subversion of the First Crusade', in Crusading and Pilgrimage in the Norman World, ed. by Hurlock, Kathryn and Oldfield, Paul (Suffolk: Boydell and Brewer, 2015), pp. 31-48.

Harris, Jonathan, Byzantium and the Crusades (London: Bloomsbury Academic, 2003).

France, John, Victory in the East: A Military History of the First Crusade (Cambridge: Cambridge University Press, 1994).

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