Shambara and the ninety-nine forts

The Rig Veda names Shambara, a powerful Dasyu chief who held 99 forts in the hills between the Beas and the Yamuna. He was defeated after a forty-year struggle by Divodasa, the Aryan king, in the battle sometimes located at Udbhraj.

This is the earliest literary reference to the territory of Himachal Pradesh and is asked almost every year.