Little is known about the transition period of around three centuries from the end of the Sangam age (300 AD) to that in which the Pandyas and Pallavas dominate the Tamil country. An obscure dynasty, the Kalabhras, invaded the Tamil country, displaced the existing kingdoms and ruled for around three centuries. They were displaced by the Pallavas and the Pandyas in the sixth century AD.
Kalabhra interregnum is called as 'dark period' because it is so called by earliest Pallava and Medieval Pandya sources. While the sangam period shows many religions entering from the North into Tamilakam (Kerala+TN), no religion was that dominant and varnashrama system was not at all fully entrenched. Jains did yeoman service to foster Tamil grammar. 95% of the earliest Tamil inscrptions called Tamil Brahmi are for the sake of Jain ascetics. These inscriptions are very short, one or two lines only.
Kalappirar are Jains, and appear to have come from Karnataka, a Jain strong hold. During the Kalabhra period, heterodoxy, opposed to Brahmanical orthodoxy, reigned supreme. Pallavas and Medieval Pandyas, who accepted the varnashrama and Brahmanical orthodoxy, completely routed out the "heterodox" religions. Both Buddhism and Jainism were practically extinguished. Hence, these "orthodox" sources portray Kalabhra period as "dark period".