Don't Ask, My Dad Has Been Lying in a Coffin for Twenty Years

The day my grandfather died, he told me three things: First, our family has been guarding a coffin for generations, sinking under the Qingjiang River. Second, my dad laid in it himself twenty years ago, for me. Third, the fingerprint on the back of my neck... Was left by the thing in the coffin. Grandpa died after finishing speaking. I was left with half a rusty iron nail, a secret book that no one can understand, and one sentence: "Take your dad home."