Demetrios Laskaris Leontares Demetrios Laskaris Leontares or Leontarios ( , died 6 September 1431 ) was an important Byzantine statesman and military leader of the early 15th century , serving under the emperors Manuel II Palaiologos ( r . 1391 – 1425 ) and John VIII Palaiologos ( r . 1425 – 1448 ) . Biography . Nothing is known of Leontares ' s early life , except a statement by the historian Doukas that Leontares had served with distinction as an officer in the Morea and Thessaly in the 1390s . Leontares first appears in the historical sources in 1403 . As a close friend and trusted agent of Manuel II , he escorted John VII Palaiologos to Thessalonica , the Byzantine Empire ' s second city , which John VII had been granted as a semi - independent appanage by Manuel . Leontares remained in the city and served as John VII ' s advisor and liaison to Manuel II until the former ' s death in 1408 . At this point , Manuel placed him as tutor and regent over his young son , the " despotes " Andronikos Palaiologos , who succeeded John VII as governor of Thessalonica . Leontares remained in Thessalonica as its effective governor until 1415 / 1416 , when the Ottoman prince Düzmece Mustafa fled to the city after a failed attempt to seize the Ottomans ' European domains from Mehmed I ( r . 1413 – 1421 ) . Mehmed demanded the surrender of the rebel , but Leontares referred him to Manuel II in Constantinople . Eventually , a deal was reached whereby Mustafa would be kept in exile in the Byzantine island of Lemnos , in exchange for an annual subsidy of 300 , 000 aspers . Leontares himself escorted Mustafa on the ship to Constantinople . In late 1420 / early 1421 and again in May 1421 , Leontares was dispatched by Manuel II to Mehmed I in missions of good will , as reports of preparations for an Ottoman attack on Constantinople increased . The first meeting occurred during a crossing of the Bosphorus by Mehmed . Leontares , at the head of a group of Byzantine aristocrats and officials and supplied with gifts , met Mehmed at the Constantinopolitan suburb of Koutoulos ( probably modern Kurtuluş ) and escorted him to Diplokionion ( modern Beşiktaş ) , where the Emperor and his sons awaited in their galley . The second meeting was a full embassy to the Sultan ' s residence in Adrianople in May 1421 , perhaps connected with a purported amendment in Mehmed ' s will that would have made Manuel guardian of his two younger sons . Leontares was cordially received , but negotiations were prevented by Mehmed ' s death on 21 May . To prevent another struggle for succession , his death was kept secret for a while , and Leontares found himself under virtual house arrest . Only with difficulty did he manage to find out news of the Sultan ' s death and report them to Constantinople . Following the death of Mehmed , the hawkish party in the Byzantine court , headed by Manuel II ' s son and co - emperor John VIII , became ascendant , and Manuel II resigned effective control of the state to him . Leontares released Mustafa from his exile , and together with his supporter Cüneyt Bey of Aydın , the Ottoman prince was promised imperial support against Murad II if he would surrender the strategically important fortress of Gallipoli . With Byzantine support , Mustafa besieged and took Gallipoli , and managed to quickly establish his authority over the Ottomans ' European domains . However , when Leontares was dispatched to demand Gallipoli from Mustafa , he was rebuffed . In the event , Mustafa was defeated by Murad in spring 1422 , abandoned by his supporters , caught and executed . Leontares appears again in 1427 , when he headed the Byzantine fleet in its last naval victory , over the forces of Carlo I Tocco ( r . 1411 – 1429 ) at the Battle of the Echinades . He later retired to a monastery , under the monastic name Daniel , and died probably on 6 September 1431 . He was buried at the Petra Monastery in Constantinople . Markos Eugenikos and Gennadios Scholarios wrote funeral eulogies in is honour . Family . Little is known about his family , except that he had brothers and was married , having one known son , John Laskaris Leontares .