Before Piett was the Admiral of Death Squadron, he was captain of the Accuser. And once Darth Vader learned of his son and realized he would someday need to take the Emperor down, he began looking for an admiral he could trust... Or how Piett got the job of captain of the Executor and eventually became admiral of the Death Squadron. This is in my Allegiance-verse, but it's not necessary to read Allegiance to understand this.