Pope Gregory IX persuaded him to pursue the heretics in his provinces and in the adjacent territories; therefore he invaded and occupied Bosnia and Zachlumia but he could not wind up Bogomilism. He supported the establishment of the Diocese of Bosnia and he granted Đakovo () to its bishop. When he was informed that the Mongols invaded the kingdom, he joined his brother's troops. However, their troops were defeated at the Battle of Mohi (11 April 1241). Coloman suffered serious wounds and died of his injuries a few weeks after the battle.