From the mid-14th century, the duchies entered a period of decline: most of the Thessalian possessions were lost to the Serbian Empire, internal dissensions arose, along with the menace of Turkish piracy in the Aegean and the onset of Ottoman expansion in the Balkans.

The Almogavars of the Catalan Company, who had recently conquered most of the Duchy of Athens to the south of Thessaly, took advantage of the situation to push north.

Led by Alfonso Fadrique, the Catalans took Neopatras in 1319, and by 1325 had also conquered Zetounion, Loidoriki, Siderokastron and Vitrinitsa, as well as—apparently briefly—Domokos, Gardiki and Pharsalus.

[2][3][4] The central and northern part of Thessaly remained in Greek hands under a series of local magnates, some of whom recognized Byzantine suzerainty, like Stephen Gabrielopoulos of Trikala; others, however, like the Maliasenos family around Volos, turned to the Catalans for support.

[10] In 1378–79, the Aragonese lost most of their possessions in Boeotia to the Navarrese Company, while from the south the ambitious Florentine adventurer Nerio Acciaioli, lord of Corinth, took over Megara in 1374 and began applying pressure on Athens.