Complex sentence: Between the 3rd and 7th centuries CE, the three Tamil kingdoms were overwhelmed by the Kalabhras which is sometimes referred to as the "Dark Age" in Tamil history and little is known about it.
Split into simpler sentences: Between the 3rd and 7th centuries CE, the three Tamil kingdoms were overwhelmed by the Kalabhras. The period of their rule is sometimes referred to as the "Dark Age" in Tamil history and little is known about it.