Boril was deprived of his two principal allies by 1217, as Latin Emperor Henry died in July 1216, and Andrew II left Hungary to lead a crusade to the Holy Land in 1217; this position of weakness enabled his cousin, Ivan Asen, to invade Bulgaria. As a result of the growing discontent with his policy, Boril was overthrown in 1218 by Ivan Asen II, son of Ivan Asen I, who had lived in exile after Kaloyan's death.

Boril was beaten by Ivan Asen in battle, and forced to withdraw to Tarnovo, which Ivan's troops laid siege to. The Byzantine historian, George Akropolites, stated that the siege lasted "for seven years", however most modern historians believe that it was actually seven months. After Ivan Asen's troops seized the town in 1218, Boril attempted to flee, but was captured and blinded. No further information was recorded about Boril's fate.