A great deal of his rule was preoccupied with fighting against the Albanians of the Despotate of Arta to his south, the Zenebishi family to his north and the Mazreku and other clans to his northwest.

Thomas gave himself the self-styled epithet of Albanian-slayer[1] (Greek: Αλβανοκτόνος[2]/Αλβανιτόκτονος[3]) after torturing Albanian prisoners in order to terrify his enemies.

[2][3] Thomas was the son of caesar Gregorios Preljub (Greek: Grēgorios Prealimpos), the Serbian governor of Thessaly, who died in late 1355 or early 1356.

After the violent death of his father, Thomas' claim to Thessaly was asserted by his mother Irene, but they were forced to flee to Serbia by the advance of Nikephoros II Orsini in 1356.

Over the next several years, Simeon Uroš recognized that he could not assert effective authority over most of Epirus and delegated power in Arta and Angelokastron to local Albanian lords.

In 1366 the citizens of Ioannina, the last major fortress to remain under Simeon Uroš's control, sent him a petition to appoint a governor who could protect them from the raids of Albanian nobles.

[7][8] The Chronicle depicts him as a cruel and capricious tyrant, describing him as "wicked", a "merciless sadist" and a "murderer", "greedy and avaricious" and special mention is made to his tyrannical actions towards the church, the nobility and regular citizens of Ioannina.

[10] Among the unpopular initiatives he is said to have taken were the intolerable taxation, the confiscation of church property as well as the imposition of monopolies on various commodities, including fish, cheese, vegetables and fruit.

In 1382 a new appointee to the local archbishopric, Matthew, was sent out from Constantinople, and invested Thomas with the title of despotes on behalf of the Byzantine Emperor John V Palaiologos.

However, several of his initiatives were essential for the financial support of his almost continuous warfare against the Albanians including the construction of defensive works and the payment of armed units.

[12] On the other hand the first secretary Manuel Philanthropenos was poisoned and the Prokathemenos of Ioannina was imprisoned, then exiled and finally murdered for alleged treason.

[13] Under Peter Losha, the despot of Arta, the Albanian Mazaraki and Malakasioi tribes attacked and at times blockaded Ioannina during the first three years of Thomas' reign.

[23][24] His assassination happened at dawn, five hours in the morning when he was stabbed to death by his guards Nikephorakes, Rainakes, Artabastos and Anton the Frank, according to the Chronicle of Ioannina.