In 1303 Roger de Flor offered the services of his Company to the Byzantine emperor Andronicus II and his son the Basileus Michael. Byzantium had been defeated by Arab armies, and was under the constant attack of the Turks advancing over Anatolia. Roger de Flor had been a Templar knight and fought to defend by sea the town of Saint John of Acre in Palestine before it was lost in 1291 to the Mameluks. Roger de Flor's offer was promptly accepted by both Byzantium and the Catalonian rulers in Sicily and southern Italy eager to rid themselves of unruly armies. Roger de Flor formed his Company with 1500 Catalonian knights and 4000 Almogavares, foot soldiers from Catalonia and Aragon.
The Almogavares were a key feature of the war between Christian kingdoms of Spain and the Muslim south. In small squads the Almogavares used to raze and sack the land of their enemies, and were frequently drafted by Christian armies to fight the Moors.
Roger de Flor arrived in Constantinople with the help of king Frederick III of Sicily in 1303, married the niece of Andronicus, daughter of the tsar of Bulgaria, and was named megaduke (head of the fleet), as was previously agreed. Roger de Flor took his Company to Anatolia where he defeated the Ottoman Turks and was given the region without the cities as his fiefdom. The unruliness of the Almogavares set free in Anatolia, and the growing distrust of the Byzantine Emperor and the Basileus who found themselves with a Western army in the heart of their empire marked the end of Roger de Flor. On April 4, 1305 he and a group of his Catalonian knights were massacred by the Alanians, also mercenaries at the service of the emperors, in Adrianopolis (now Edirne) while attending a banquet offer in their honor by the Basileus Michael. The king then marched over to attack and take Gallipoli from the Company under Berenguer de Entenza. The attack was unsuccessful, but it decimated the Catalonians. Berenguer de Entenza was captured by the Genoese shortly after, and later liberated. The Company had 206 horsemen, 1256 foot soldiers left and no clear leader when king Michael, trusting in his superiority in numbers, attacked again only to be defeated in Apros in July 1305.
The killing of Roger de Flor and his group unleashed what was to be called the "Catalonian Revenge" against the Byzantine Empire. Following the rules of chilvary, they challenged the Emperor himself whose only answer was to kill their embassies, and ended up killing every Catalonian and Aragonian living then in Constantinople. The Company answer was to devastate the regions of Thrace and Macedonia for the following two years - devastation that stayed in Greek popular memory until the beginning of the 20th century.
The Company was then a powerful prize in imperial politics that Frederick III of Sicily tried to control. Frederick III sent the Infante Ferran of Mallorca to Gallipoli as Captain. However, Bernat de Rocafort, one of the leaders of the Company, opposed this move and faced Berenguer de Entenza, Ferran Ximenis d'Arenós and others that accepted the Infante. The struggle ended with the departure of the Infante Ferran and the Company under the control of Bernat de Rocafort. The administrator, Ramón Muntaner also left then and later wrote the chronicles that tell these stories. Bernat de Rocafort offered the services of the Company to Charles of Valois to help in his claims to the Byzantine Empire and whom the Catalonians had expelled from Sicily before founding the Company in a dynastic war for the crown of Catalonia-Aragon. In 1309, Thibault de Chepoy, the deputy of Charles of Valois, ended the tyrannical rule of Rocafort seizing him and sending him to Naples where he died of hunger the same year.
The Company offered its service to Walter of Brienne, Duke of Athens, in 1310, and within a year it freed the duchy of its enemies, only to be betrayed by Brienne who did not recognize his obligations. The Company attacked, defeated and killed Brienne on March 15, 1311 in the battle of Halmyros on the river Cephissus in Boeotia, and thus took control of the duchy of Athens.
No longer under control of the Franks the new Catalonian lands expand into Thessaly and become the duchies of Athens and Neopatria until 1388-1390 when they fell to the Florentines under Nerio Acciajuoli of Corinth. His descendants controlled them until 1456 when they were conquered by the Ottoman Empire. By that time, like many military adventures the Great Company had fizzed out of history.