Task: Question Answering
Input (English): Passage: After the conquest of Bulgaria, Basil II prevented revolts by retaining the rule of local nobility and relieving their lands of the obligation to pay taxes in gold, allowing tax in kind instead. The Bulgarian Patriarchate was reduced to an archbishopric, but retained its autocephalous status and its dioceses. Byzantine domestic policies changed after Basil's death and a series of unsuccessful rebellions broke out, the largest being led by Peter Delyan. In 1185 Asen dynasty nobles Ivan Asen I and Peter IV organized a major uprising which resulted in the re-establishment of the Bulgarian state. Ivan Asen and Peter laid the foundations of the Second Bulgarian Empire with Tarnovo as the capital.
Kaloyan, the third of the Asen monarchs, extended his dominion to Belgrade and Ohrid. He acknowledged the spiritual supremacy of the pope and received a royal crown from a papal legate. The empire reached its zenith under Ivan Asen II (1218–1241), when its borders expanded as far as the coast of Albania, Serbia and Epirus, while commerce and culture flourished. Ivan Asen's rule was also marked by a shift away from Rome in religious matters.The Asen dynasty became extinct in 1257. Internal conflicts and incessant Byzantine and Hungarian attacks followed, enabling the Mongols to establish suzerainty over the weakened Bulgarian state. In 1277, swineherd Ivaylo led a great peasant revolt that chased the Mongols out of Bulgaria and briefly made him emperor. He was overthrown in 1280 by the feudal landlords, whose factional conflicts caused the Second Bulgarian Empire to disintegrate into small feudal dominions by the 14th century. These fragmented rump states—two tsardoms at Vidin and Tarnovo and the Despotate of Dobrudzha—became easy prey for a new threat arriving from the Southeast: the Ottoman Turks. 
Question: What ruler was overthrown by a group whose factional conflicts caused the Second Bulgarian Empire to disintegrate?
Output (English): Ivaylo.