The Secret of the Nagas — Amish Tripathi

At a crucial moment after the cliffhanger ending of book one, The Immortals of Meluha, the warrior-hero Shiva--the man who is the prophesied Neelkanth, or destroyer of evil--is fighting to protect his wife Sati from an invasion by the Nagas, a mysterious militaristic society.
Evil forces are growing in strength all around the conflict, and Shiva's own philosopher-guides have betrayed his unquestioning faith by accepting aid from the dark side. Even the idyllic empire of Meluha hides a terrible secret.