Battle of Mosynopolis

While Kaloyan's army besieged Edirne from the east, the king of Thessalonica, Boniface of Montferrat, decided to attack Bulgaria from the west with Byzantine support. His cavalry successfully reached Mosynopolis after a five-day raid from Serres. In the mountainous area around the town, however, Boniface's army was attacked by a larger force composed mainly of local Bulgarians.

The fighting began among the detachments on the flanks, where Boniface managed to drive back the Bulgarians. While pursuing the enemy, however, he was wounded by an arrow, and his knights were defeated soon afterwards. The king's head was sent to Kaloyan, who immediately organised a campaign against Boniface's capital, Thessalonica. During the siege, however, Kaloyan was killed by conspirators, leading to the withdrawal of the Bulgarian troops and the lifting of the siege.