Complex sentence: The Sassanid Empire, lasting the length of the Late Antiquity period, is considered to be one of Iran's most important and influential historical periods, and was the last great Iranian Empire before the Muslim conquest and adoption of Islam.
Split into simpler sentences: The Sassanid Empire, lasting the length of the Late Antiquity period, is considered to be one of Iran's most important and influential historical periods. In many ways the Sassanid period witnessed the highest achievement of Persian civilization, and constituted the last great Iranian Empire before the Muslim conquest and adoption of Islam.