Complex sentence: Chormaqan and the majority of his army then entered the northern section of Persia known as Mazandaran in the autumn of 1230, thus avoiding the mountainous area south of the Caspian Sea.
Split into simpler sentences: Chormaqan and the majority of his army then entered the northern section of Persia known as Mazandaran in the autumn of 1230. In doing so, he avoided the mountainous area south of the Caspian Sea.