Oath of the Vayuputras (Shiva Trilogy: Book 3)
This is the third book in the Shiva Trilogy (which includes Book 1: The Immortals of Meluha and Book 2: Secret of the Nagas).
Shiva has accepted his destiny, but it has brought him to despair.
Books on the ancient Indus Valley people by fiction author Amish Tripathi.