Complex sentence: Saladin continued his hostility to the remaining Zengid power in northern Syria and Upper Mesopotamia until 1186, when hostilities ended upon Izz ad-Din Mas'ud submitting and agreeing to become Saladin's vassal.
Split into simpler sentences: Saladin continued his hostility to the remaining Zengid power in northern Syria and Upper Mesopotamia until 1186, when hostilities ended. Peace was made upon the submission of Izz ad-Din Mas'ud, who agreed to become Saladin's vassal.