Send Me as a Hostage, Why Are You Crying When the Army Invades?

Tang Cheng guarded the frontier for seven years, and his achievements, large and small, filled half the room. But just because he was the product of a marriage, he became an obstacle to the prince. He was even sent to a neighboring country by his biological father to be used as a hostage. I thought I would only suffer humiliation in the grassland for the rest of my life, but suddenly there was news from my grandfather whom I had never met before. "My dear grandson, do you want to be the emperor?"