In 1256 Kaliman Asen murdered his first cousin Michael Asen I during a hunting party in the environs of the capital Tărnovo and usurped the throne. In the process he married Michael Asen I's widow, the unnamed daughter of Rostislav Mihailovich, but he was unable to maintain himself on the throne. Rostislav advanced on Tărnovo from Belgrade, and Kaliman Asen II fled the capital. Rostislav returned home with his daughter and claiming the title of emperor of Bulgaria, while a brother-in-law of Michael Asen I, Mitso Asen took the throne.
In the meantime, Kaliman Asen II had been murdered after being abandoned by most of his supporters.