How did Mongol attacks affect the second Bulgarian empire?

During the era of the Second Bulgarian Empire, Mongol attacks from the north served as a significant destabilizing force that contributed to the state's decline following the height of its power under Tsar Ivan Asen II.21

These external military pressures, combined with internal upheavals—such as the growing burdens placed on the peasantry by powerful nobles and the resulting peasant revolts—eroded the central authority of the Bulgarian tsars. As the Asen dynasty lost its grip on power and eventually died out in 1280, the subsequent ruling houses were largely unsuccessful in restoring the empire's former strength. Ultimately, these cumulative factors left the state vulnerable to further encroachment and disintegration, eventually leading to its fall to the Ottoman Turks.213