The Art of Destiny, War Arts (02) by Wesley Chu

A hero once believed to be the chosen one must find a new path with the help of a band of unlikely allies in the sequel to The Art of Prophecy, an epic fantasy ode to martial arts and magic from the #1 New York Times bestselling author Wesley Chu.
Once there was a prophecy that a chosen one would rise to defeat the Eternal Khan, an immortal god-king.
But the prophecy was wrong.
Now Jian, the former chosen hero, is just an ordinary young man trying to find his own way. But he may yet have an extraordinary destiny, because he joins forces with Taishi, his grumpy grandmaster, who instructs him in the ways of her family's powerful war art. Jian still has a long way to go before he can become her heir, so she recruits a band of elderly grandmasters who come out of retirement to whip him into shape and help with this one last job.
And there are others who are also seeking their own destiny, like Qisami, an assassin on a secret mission...