There are several references to the cathedral in medieval sources . The earliest reference to the church tell of the transfer of Saint Michael the Warrior 's relics from the Potuka fortress to the Patriarchal Cathedral on the order of Tsar Kaloyan ( r . 1197 – 1207 ) . The housing of a warrior saint 's relics in the Patriarchal Cathedral signify the incessant warfare against Byzantines and Latins that dominated Kaloyan 's reign . In the late 14th century , the last Patriarch of Tarnovo , Saint Evtimiy , described the church as the " great patriarch 's Cathedral of the Holy Ascension " in his writings .