Prince Harry wrote about his 2005 Nazi costume in his leaked memoir Spare.
Prince William and Kate reportedly told Harry to wear the Nazi uniform.
The royal couple reportedly laughed when they saw him in the costume, causing trouble.
Olympic athlete Richard Meade threw the Duke of Sussex's son Harry's birthday party.
Soon after, the British royal family apologised, saying that the clothes were a bad choice.
He later talked about what happened in his new Netflix show with his wife, Meghan Markle.
Harry met with London's Chief Rabbi and a Holocaust survivor in Berlin to learn from his mistakes.
Before it comes out, the upcoming memoir, Spare, is being carefully watched over.
Its security is said to be like that of "Harry Potter and the Deathly Hallows," which came out in 2007.